speedracer1407

I see what you're saying, OSUITS. I suppose it does make sense to have a simple hydraulic system powering a primary engine cooling fan, as they certainly are large and the plumbing is short. And the diesel engine in all of the CTA's NFs is the same cummins 10.8L ISM or 8.9L ISL. What I had in mind, however, is that the offending fan is not a primary engine cooling fan, but rather, one of the many fans associated with the electric drive/storage system. At this point, it's total speculation, as I'm not familiar with the specific architecture of the hybrid drive, but the offending fan in the video spools up and down independent of load and RPM. As the electric drive and storage system constantly shuttles electric power around the system, from the motors to the batteries, etc, it seems to make sense that this fan cools a component of that system, not diesel engine. These fans are likely much smaller, and since there's so many of them, it's unlikely they're all powered by a hydraulic system. Wouldn't it be great if there were a Cummins or NF engineer hanging around on this forum?

Fans powered by hydraulics? I'm skeptical about that that for 2 reasons: Firstly, a hydraulic system powering multiple components (there are a lot drivetrain-related fans on these artics) is inherently way more complicated and prone to mechanical failure than electric fans. This seems like a really weird engineering solution, especially in a drivetrain with robust electric capabilities. Secondly, in the 2nd video, the failed fan component (if that's what it is) spools up and down repeatedly during steady-state acceleration. If, as you say, hydraulic fans are dependent on engine speed, the offending fan would be producing its awful noise more steadily, since you can also hear that the engine is under steady load and RPM while accelerating and maintaining speed on LSD. And even if the engine RPM rose and fell during steady cruising (which doesn't seem to be the case in the video), it certainly wouldn't do so as much as that noisy fan seems to. Hydraulic fans that scale with engine speed are news to me. Doesn't mean there isn't a hydraulic component to their actuation, but I'd be really surprised if they were operated entirely with hydraulics AND scaled with engine speed.

Wow, that sounds like it would make for an irritating ride to work. If it's a fan component, which it sounds like it is, I wouldn't necessarily call it a "defect." Just one of many components that wear out and fail from time to time. As a side note, I certainly miss flying down LSD on the 147 at top speed, knowing that I was making rapid progress to the office. Now it's the Red Line, which doesn't make rapid progress anywhere.

Yes, the artics make some odd noises at seemingly random times, but if you ride them frequently, as I did every day until recently, it's pretty easy to learn the noises' patterns and when to expect them. This is definitely not normal, thought it isn't necessarily an impending mechanical disaster. It's clear that, in the video, the bus was accelerating from a stop, and the volume/tone of the noise did not scale with the acceleration of the bus, so it's probably not anything related to internal transmission or engine components. However, the artics have a chorus of what sound like fans that kick on and off depending on speed and load while accelerating (and then an even louder chorus during brake regen). I'd wager a reasonable sum that this noise is a cooling fan component (bearing or motor) that's failing. Here's why: The artics have a predictable sequence of noises when accelerating off the line. The first few seconds is all electric power. By 10 MPH or so, the diesel wakes up and begins adding power gradually (listen carefully, and you can hear the turbo spool up early-on). By 20 MPH or so, the diesel is working under high load, and various cooling fans (or what sounds like fans, anyway) spool up and stay spooled until the driver lifts off the pedal, at which point both the diesel and the fans go relatively quiet. Seems to me like this sequence describes the noise in the video with a faulty fan component (or whatever it actually is).

I can't speak for the specific operation of CTA equipment, but generally speaking, fast idle is needed to power heavy-duty accessories like, say, a wheelchair lift. As far as I know, most wheel chair lifts (the kind that actually lift the wheelchair and its occupant up to bus-level, are hydraulically powered. The hydraulic system is powered by a power take off (PTO), which is a geared shaft usually connected to the engine near the flywheel or to the transmission. This shaft drives the hydraulic pumps that pressurize the system. To feed the hydraulic system with enough pressure during high-load usage, the engine must run at a higher idle (fast idle) than normal to spin the PTO shaft fast enough and/or with enough force. Hope that helps.

Yes, it's my understanding that, in order to coast, the driver must maintain slight pressure on the pedal. This doesn't seem intuitive at all, and I'd imagine it's frustrating for drivers used to a non-hybrid bus. I ride the 147 every day, and have observed time and again the driver taking his/her foot completely off the pedals, allowing the bus to slow rather quickly the full length of the Foster exit ramp, and only applying the service brakes in the final few seconds. Sometimes, drivers are less than delicate with the pedals, and while trying to maintain a constant speed over overpasses on LSD, they end up engaging brake regen over and over on the downhill portion of the overpass without ever touching the brake pedal.

I'm not an Allison engineer or anything, but that "humming" noise is likely a fan or series of fans cooling the electric components of the regenerative braking system. And it doesn't happen only during an abrupt stop. As far as I can tell from observation, anytime the driver lifts his or her foot off the gas pedal, brake regeneration kicks in. Contrary to what some may assume, brake regen has nothing to do with the actual brakes. Under acceleration, the electric motors inside the transmission are used to assist by drawing current from the batteries. But by reversing the flow of electricity, the electric motors can act as generators, thus charging the batteries. When this happens, the motor/generators provide a drag on the driveline that's roughly similar to their accelerative capability, and the result is some pretty effective braking. This is brake regeneration, and in these buses, it seems like it's programmed to activate any time the driver takes his or her foot off the gas. It's a win-win scenario: The bus slows down, saving the service (friction) brakes, and at the same time, the electric motors generate electricity to charge the batteries. Cooling fans are necessary to keep the components from overheating during the spike power transfer, and that's what you hear when a 4000 series slows down.

To each his own, i guess. But I've had a distinctly different experience with these buses, especially sitting in the back. I've been riding the 147 for near 2 years now, round trip every day, starting with the horrible NABIs and finding relief in the 4000s The front (as in near the front axle) is acceptably smooth, but the air springs are calibrated stiffly enough to transmit pretty severe jolts over sharp bumps. And more stuff seems to rattle up there, so I feel like the front is not the ideal spot. The middle is the worst. Those 5 or so side-facing seats over the middle axle are positively punishing over rough roads. For whatever reason, maybe low floor and lack of suspension travel (thus stiffer air spring settings), sitting in the middle makes me feel like I have jiggly man-boobs, and I'm a rather slight 155lb. Sharp frost-heave bumps are great for compressing the spine. The back, near the rear-most axle, is the most comfortable by far in my experience. Yes, it's loud, and the brake regeneration cooling fans are annoyingly loud, but I suspect the suspension has a LOT more travel in the high-floor section. Sharp bumps that will blur my vision in the front and middle of the bus are mere vibrations in the rear. Make no mistake, thought; on a bumpy road, the rear section is a very giggly ride. It's the only driven axle, so it has a big, very heavy differential and drive shafts built into the axle. The axle assembly probably weights several hundred pounds more than the other axles, so it bounces around under the chassis more. But those softly calibrated air springs in the back make for a relatively impact-free ride. Having ridden in nearly every seat on North Park's 400s, I'll take the noisy back every single time if I can. As for being bulky and slow. Well, yeah, they're 60ft artics, so they're big. And while I've commented on this forum before about them being astonishingly slow to respond to throttle inputs, once under way, they seem to accelerate briskly, especially from 5-30 MPH or so. One universal complaint I have about these buses, however, is the awful new seats. Nobody like sitting sideways on a bus that's constantly jerking between acceleration and braking. And the seats are designed for people with cone-shaped butts. I'm not big at all, and yet I feel like I'm sitting in a child's chair.

While the diesel in a series hybrid like the ISE system does indeed spend much of its time charging the batteries, it's capable of more than that. Unlike a parallel hybrid, the engine doesn't directly interface the drive wheels via a mechanical transmission, which you probably already know. Instead, it turns a generator that can either charge batteries or energize the electric drive motor(s). Ideally, the batteries will never have the change to discharge enough that the engine is left to do all the work, but should that happen, all of the generator's power, generated by the engine, will go directly to the electric drive motors.

On a spec sheet for transit bus duty, they look pretty similar, but they actually don't share any design lineage with each other, so it's not like one is a bigger/smaller version of the other. The ISM is a 10.8 Liter Inline 6 rated at 280HP and 925 lb/ft of torque in the 40 footers. It's also available with 330HP and 1150 lb/ft of torque, presumably for 60ft artics. The ISL is a 8.9 Liter Inline 6 that's also rated at 280 HP and a slightly lower 900 lb/ft of torque. Again, 330HP and 1100 lb/ft torque rating is avaiable for larger buses, and according to New Flyer's literature, the Hybrid artics have the 330hp version. The ISL is simply a modernized and slightly enlarged version of the C8.3, which itself received modern electronics, emissions, and common rail direct fuel injection to become the ISC. I recall seeing on this forum that the Nova's use the C series engine. All three engines are capable of delivering similar HP and Torque, so it begs the question: why offer three engines all capable of doing the same thing? Well all of these engines are capable of, and are rated at, much higher HP and torque in heavier-duty applications, like fire trucks. And that's where the differences are apparent. The ISM can top 500HP, the ISL over 400, and the ISC up to 365. So essentially, they're all "detuned" substantially from their potential maximum output when used in transit buses, some more than others. I presume this for the sake of longevity; a fire truck engine that only covers a few miles a day can afford to blast out 500 HP when needed. But when it comes to a stop-and-go cycle all day every day for a decade, it makes sense to use that same engine, derated to 280HP, for easy, under-stressed running. So the CTA seems to have settled on the ISL, which intuitively makes sense. The too-big ISM is wasteful, the ISC might be overstressed for the long-haul, so the ISL is just right. I donno if any of that is true, but it makes sense to me.

I suppose it's worth noting that, for the engine key, the 4000s indeed have the "GM/Allison Hybrid drive. A silly distinction, perhaps, but it's really a transmission with integrated electric motors. The engine is a Cummins ISL diesel that, according to New Flyer's literature, is rated at 330 HP. I'm pretty sure the Diesel on the 800s is the smaller Cummins ISB, but I'm not totally sure about that.

Various engine defects can cause hissing and whistling, but what you're likely hearing is the turbocharger operating normally. Virtually all 4 stroke Diesel engines in transport applications have turbos (and have for decades). You may already know this, but a turbocharger is a centrifugal compressor that compresses the intake air, thus delivering a denser charge of air into the combustion chamber, allowing more fuel to be pumped in for more power. It works as follows (and pardon me if you're already familiar): Exhaust gasses exit the engine very hot, at high speed, and under high pressure. The turbo sits in the path of the exhaust, and the rushing gas blows onto a turbine, causing it to turn very fast, up to 100,000 RPM. The mechanics are exactly the same as blowing really hard on a pinwheel. After blowing through the turbine, the exhaust exits through a pipe and out the exhaust stack. The turbine, however, is connected via a shaft to a compressor. When you accelerate, the exhaust pressure builds and "spools up" the turbine, turning the shaft, which turns a compressor. Fresh intake air is routed through this compressor before reaching the engine. It's compressed, passed through an intercooler (like a radiator to cool it down), and fed into engine as cool, high-pressure air, which can "absorb" more fuel, and thus allow the engine to make more power. The whistle you're hearing is the sound of the turbine and compressor blades whipping through the surrounding pressurized air at super high speed. The CTA's DD50s in the 6000s, 5800s, and 7500s all had turbos that were quite loud. In particular the 5800 was just about deafening when sitting in the rear-most seats, but delightful for engineering dorks like me. (I'm not an engineer, just an engineering dork). For some reason the turbo is harder to hear on ISM and especially ISL powered 1000s. But if you listen carefully, and you probably can't hear it from the driver's seat, ISM-powered 1000s have really quick-spooling turbos. Whereas DD50s, especially those in 6000s and 5800s, have turbos that take ages to spool, the ISM has a variable geometry turbo that restricts incoming exhaust to the turbine at low RPM when exhaust pressure is low, causing a artificially high presser on the turbine blades, causing faster spool-up. Sorry if this was all old news for you, but it's fun stuff to know for gear-heads.

I'm not so sure it's really that the available power from the electric assist is chronically inadequate to get the bus moving and/or reduce reliance on diesel power. It seems to me like it's more about the way this exceptionally sophisticated drivetrain system is programmed. You're right that electric motors can provide instantaneous torque; in fact, they almost always produce their maximum torque at 0 RPM. But the "problem" here (aside from the odd failure-like behavior that I started this thread with) seems to be that the programming allows for a huge delay between the driver's pedal input and the drivetrain computer's response. As I noted before (or maybe in another thread), once underway and working properly, these buses don't feel slow to me. For the first 10-15 MPH, the Diesel stays relatively dormant, yet the electric motors provide a respectable shove of torque. After that, the diesel begins feeding in supplementary power, eventually reaching high-revs as we pass through 40 MPH or so. The trick, it seems, it to avoid lifting off the pedal, because getting back on the power begins the waiting game all over again. It makes sense to program the drivetrain to feed power gradually. Consider this: The 8.9L ISL in these makes the same 330HP (and 1100 lb/ft of torque) as in the non-hybrid D60LF, according to NewFlyer's literature. And the transmission's electric motors provide, well I donno how much, but it's enough torque to move the thing at a good clip off the line once it finally gets around to it. Drivetrain longevity depends on gradual application of this available torque, and it seems like the CTA's 4000's go far beyond a "sensible" application.

Thanks for the detailed answer! I still wonder, though, why there's a pneumatic component to the transmission. The retarder is hydraulic, forcing oil through a series of rotors and stators, which puts drag on a shaft, which transmits that drag to the output shaft of the transmission.

Thanks for the reply. What you say makes sense in day-to-day driving, and I witness this every day on the 147 trying to accelerate up entrance ramps or accelerating out of slow traffic and climbing over overpasses on LSD. Indeed, the higher the demand for acceleration, the higher the RPMs from the Diesel. But what separates the usual surge of Diesel RPMs to augment electric power and this particular instance is this: At the time we were going maybe 20 MPH and accelerating up the ramp. This is below the speed at which the Diesel usually begins revving hard (It's still 'blended' about 1/2 and 1/2 with electric power at this point). Then, MUCH more suddenly than if the drive had simply lifted off the gas, the bus gave a small shudder and acceleration stopped abruptly. Then with barely a beat's delay, the Diesel RPMs shot up WAY more quickly than normal, going from low to High RPM in a second or two, and acceleration was restored. As a passenger, I can't say I've noticed a marked difference in acceleration between the first 4000s and the latest ones. But yeah, in general, they're very slow to respond. Even sitting in the back, I hear many drivers tap-tap-tapping the gas to the floor after the doors close out of frustration trying to get the thing to move. It takes several seconds just to begin moving once the pedal is to the floor, and the electric motors seem to be programmed to ramp up the torque very slowly. A few months ago, I rode a 4000 that was so packed, I had to stand next to the driver and hold bar next to the fairbox. I paid close attention to the relationship between his pedal inputs and the acceleration response and was surprised at how long it took for the thing to respond. Even though I hadn't spoken to the driver, he complained about how slow the buses were, and offered a demonstration of just how slow it was. In traffic on LSD, from a dead stop, he said "watch this," Then slammed the gas to the floor. We waited, and waited some more, before finally feeling some push. And it seems even worse trying to accelerate while already moving. On that same busride, I noticed that, after slowing to maybe 30 MPH or so for traffic, then trying to get back up to speed up hill on an overpass, the driver would floor it, and there'd be no response whatsoever from the engine, untill we'd crested the hill, lost more than 5 MPH, and started down hill. Only then would the diesel wake up and starting providing meaningful thrust. This slow accelerator response can be really annoying for passengers. I can't tell you how many times we've been caught at that ridiculously long stoplight at Bryn Mawr and Sheridan because the bus wouldn't step off fast enough after unloading passengers to make the green. I have to say though, that once underway, the 4000s will charge up to governor-limited top speed with authority.

Thanks for the reply, Busjack. Naturally, any automatic transmission (planetary, CVT, or otherwise) has the capability of "down shifting" to bring engine revs up and take advantage of a higher input/output ratio (lower gear or CVT setting) to maintain acceleration when needed. Indeed, dating back to the 50s. But my curiousity in this instance was about the somewhat unique way in which the Allison hybrid transmission is designed. The two electric motors integrated into the otherwise ordinary 4 speed (I think) transmission provide both electric boost and ratio modulation. In other words, the electric motors act as a clutch, so the engine can operate at whatever revs the drivetrain computer decides are necessary and most efficient for the situation. The net result is that, during full-throttle acceleration from low speed to top speed, the engine revs don't rise and fall with each gearshift, but rather, rise and fall depending on how much electric boost is available. And from observation, the engine revs stay don't rise and fall all that much. From about 20 MPH to near 40 or so, the engine remains at a constant RPM thanks to the CVT. On this ride, however, acceleration was abruptly cut, accompanied by a slight shudder, and then the engine revs shot up to high RPM and stayed there (not rising or falling) as we accelerated to a rather brisk speed. So the CVT effect of the electric motors was still in effect, but it seemed that the electric boost had failed. I'm pretty sure that the idea of electric motors acting as clutches to modulate engine RPM and transmission ratio isn't new, but it does require an electric power source to flow current through the motors. If, indeed, the electric system failed or ran out of battery juice, how would the clutching acting of the motors work? That was my question, though perhaps I didn't explain it thoroughly enough. Any thoughts are appreciated.

I'm pretty sure that the 4000 series I was on the other day experienced a failure of the electric portion of the drivetrain. I was on the a southbound 147 in the morning, and as we accelerated up the Foster entrance ramp to LSD, acceleration suddenly ceased. It wasn't that the driver let off the accelerator, as that is usually accompanied by a gradual spooling-down of engine revs. This time, acceleration abruptly stopped at about 30 MPH (a guess), and the diesel immediately spooled up to high-RPM-much higher than during normal acceleration at any speed, and acceleration was quickly restored. This scenario had never happen before while riding a 4000. After the long ride down LSD to 1000 Michigan, it seemed that the usual diesel-electric operation had been restored. This little story may seem irrelevant, but I came away with two things: Firstly, if this was actually a momentary failure of the electric portion of the drivetrain, it's nice to know that the diesel engine (rated at a full 330 HP) can operate independently to keep things moving at a normal pace. Secondly, How? The electric motors in the transmission also act as clutches to make the otherwise ordinary planetary gearset act as a continuously variable transmission. Without electric power, I'm a a loss for how the transmission could continue to perform as a CVT (which it did, since there were no discernable gear shifts, and engine revs spooled up high and stayed there throughout the acceleration run into LSD).

I'm somewhat surprised to hear that the Series 50 has a reputation for crapulence. Oddly, I thought the only decent thing about the CTA's horrible NABI artics was the drivetrain. For one, they never broke down on the hundreds upon hundreds of rides to and from work on the 147. For another, they were substantially smoother than the ISM and ISL engines installed on the NFs, both 1000 and 4000 series. And while they didn't exactly make the NABI artics a hot-rod, they shoved those horrible rattle-traps up to LSD speeds reasonably well. But I certainly wouldn't know what maintenance nightmares went on at the garages, whether CTA or elsewhere. But I'm not so sure the 4 cylinder design was necessarily a major cause of their unreliability. The DD50 may have been flawed, but there's no good reason why a 4 cylinder should be inherently overstressed or unreliable compared to a 6 cylinder of similar displacement and power. After all, it's not like the DD50 was undersized; it had exactly the same displacement (8.9L) as the ISL currently used in all CTA 1640+ and 4000 series buses. NF's promotional specs also name the ISL as the engine available on non-hybrid 60ft artics, making a full 330 HP, similar to the DD50 in the NABIs. Conventional wisdom may suggest that spreading 280-330HP over 6 cylinders may be more reliable than over 4 cylinders, but the reality is that neither engine revs much over 2000 RPM, so rotational and reciprocal mass is less of an issue than it would be if one were trying to extract, say, 250 high-RPM HP out of a long-stroke 2.5L 4 cylinder gas engine in a sports car vs a 2.5L V6.

After riding on a #1930+ NF equipped with the Allison B-400R transmission, and watching many pull away from the Michigan/Huron stop while waiting fo the 147, I've noticed a sound that seems to be transmission related, in part because it doesn't happen on the ZF equipped NFs. If the driver accelerates at any speed, and for any amount of time, and then lifts off the gas, a loud single blast of air is released from the rear of the bus behind the wheels. It's loud enough to startle from the outside, and it'll kick up a plume of dust in the right conditions. Normally, I'd just assume this was the result of some modification to the bus's existing pneumatic brake or suspension systems applied to 1930+ buses. But because it *always* happens at the moment the driver lifts off the pedal after accelerating, I suspect that it's unique to the Allison transmission itself. It's too consistent to be coincidence; while riding on one a few weeks ago, it happened each and every time the driver lifted off the pedal after accelerating, and only at those times. Can anyone confirm that the Allison transmission has some pneumatic component that would cause this sound?

I can't tell if you're wondering why losing pressure causes the brakes to lock, or if you're specifically asking why a bus would dump all its brake air pressure for no apparent reason. So I'll try to cover both, so we're on the same page. All air brake systems, truck, bus, or train, are designed to be "fail safe." That is, a loss of brake system air pressure will cause the brakes to engage, which either stops a moving vehicle, or prevents it from moving in the first place. On a train, moving the brake lever actually reduces pressure in the main brake pipe, causing a sliding valve to move, which opens a pathway for a separate reservoir tank to dump its pressure into the brake cylinder, which pushes a piston, which applies the brake shoe to the wheel. This system is considered "fail safe" because a leak or failure of the main pressurization system (compressor, brake pipe, etc) will cause an uncommanded brake application and will stop the train or prevent it from moving. A bus (and truck) system is similar except that the service brake application and fail safe application are separate elements of the same system. Pressing the brake pedal opens a valve, which releases air from a reservoir tank, which fills a brake cylinder, which acts on the brake shoes of a drum brake. Without a fail safe system, a loss of brake air pressure would leave the bus without any braking action. But each brake cylinder also has a separate compartment with a very stiff spring that is directly connected to the brakes. This compartment MUST be filled with pressurized air from the main brake system at all times to prevent the spring from applying the brakes. When the main brake system pressure gets too low, the spring brake compartment will also lose pressure, and the spring will be allowed toto apply the brakes and will stop the bus or prevent it from moving. I have no idea why a bus will unexpectedly empty its air when coming to a stop. I suspect, however, that a bus will never dump ALL its brake system air. Rather, I suspect that low air pressure will trigger a safety system that dumps spring break air once the bus has stopped, preventing it from moving again untill normal system pressure is restored. I hope some mechanics or others who are more knowledgeable about air brake systems (my weakest subject) can either confirm or correct what I've explained here.

I'm curious about what operators think of the hybrid artics' throttle response. On the rare occasion that I'm sitting near the front, I often hear the "clack" of the driver stomping on the accelerator from a stop. It makes a distinct noise, and some operators have the odd habit of tapping the pedal to the floor a few times in rapid succession (tap tap tap) pulling away from every stop. Anyway, 40 ft NFs seem to respond quickly, launching hard off the line. 60Ft hybrids, however seem to take forever to begin generating meaningful forward progress. I recall a few times when the operator closed the doors and stomped on the gas at the same time. Several seconds went by before torque from the electric motors gradually ramped up. It's probably hard for operators to hear what's going on in the engine compartment. But daily observations aboard 4000s on the 147 lead me to believe that the driveline computers take their sweet time getting the bus moving from a dead stop.

Probably just a typo on your part, but 1930+ don't have Allison engines, as Allison doesn't make engines. They have Allison transmissions and the same Cummins ISL that all 1640+ Newflyers do.

I'd like to see them in action. But I haven't seen any 1930+ on the 147. Does NP have any? I'm curious about whether or not the Allisons can actually maximize acceleration any better than the ZFs. To me, the ZF is perfectly geared for a transit bus, with very closely stacked ratios that get the bus moving quickly from a stop through 3rd gear.

I'm surprised that the NABIs have ZF transmissions. They behave and sound completely different from the 6 speeds in the 1000s. I suppose that shouldn't be surprising: different transmission, different year. But big commercial equipment isn't redesigned very often, so I'd be surprised (again) if the 5HP602C doesn't have the same internals (minus one ratio) as the 6 speeds in the 1000s. But I wouldn't know. Also, why did 1930-2029 switch to the Allison transmission? Did it have something to do with getting a better deal on them because of the 150+ Allison-equipped #4000s? The B400 comes in 5 and 6 speed variants, from what I can tell; which did the CTA order?

I wanted to respond to each point of your post individually, but that feature is unavailable on this forum. So I'll respond to each paragraph sequentially. I added numbers to your quoted response for ease of reference 1) "Final Drive" is not a gear inside the transmission, but rather, the final drive ratio, or the final gear reduction inside the differential. All cars, trucks, buses, etc have a final drive, though the term may vary in different transportation engineering vernaculars. #4000s on the 147 emit a pronounced gear wine that grows very loud and increases in pitch at high speed on LSD. I assume this is from a strait-cut gear component inside the differential because the pitch of the whine is directly proportional to the speed of the bus. For what it's worth, this gear whine is exactly the same on the #1000s, likely because they share a similar differential, and it too is proportional to speed rather than engine RPM. 2) I find the them to sound very much like a CVT in that the engine note (which as you point out below is indeed difficult to hear at times) rises and falls completely irrespective of vehicle speed. Rather, the engine revs up and down (rather slowly) depending on load, and in most situations, maintains a relatively constant RPM, even when accelerating. 3) Right. But I suppose it depends on how you define CVT. A "conventional" CVT, as found in the Prius, does indeed use a variable belt transmission. But the Allison transmission uses the two electric motors and clutch packs, along with the more "conventional" planetary gear sets of a standard automatic transmission to create continuouse and infinite gear ratios. Thus, I was using the term CVT because it is indeed a transmission that can continuously vary its ratios. 4) I'm a little unclear about what you mean by "in which case it wouldn't save fuel." As you noted, both the Prius and the hybrid bus are parallel hybrids, and in theory, both can operate on electric power, gas/diesel power, or both. All parallel hybrid cars shut down the gas engine when the computer decides it isn't needed, such as when accelerating slowly from a stop, coasting, decelerating, etc. in an effort to save fuel. After a few hundred miles seat time driving my friend's Prius, I came away surprised at just how often the computer shut down and restarted the engine. Sometimes, it would go on and off 4 or 5 times in a minute while cruising around town at a constant speed. In the #4000s, however, the diesel engine is always running, even when stationary. While shutting off the engine may save fuel by allowing the electric motors to keep systems running and creep the bus forward in heavy traffic, I'm willing to bet that starting and stopping an 8.9L diesel engine hundreds of times a day presents added complexity and potential reliability problems on a bus designed for 12+ years and hundreds of thousands of miles of hard use. 5) Agreed. I had a hard time figuring out what the engine was doing at first. But after riding the #4000s day in and day out for a while now, I've been getting better at distinguishing various engine compartment and HVAC noises from actual engine noise. And since the HVAC lives on the roof of the 4000s, it's a bit less intrusive in back than on the 1000s. BTW, here's a highly technical paper on various hybrid systems, including the GM/Allison. When it gets into formulas, it's way over my head. http://www.engin.umd.umich.edu/vi/w4_works.../Miller_W04.pdf