4000 series electric drive failure

[speedracer1407]

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).

[Busjack]

... 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).

It just might have been that the transmission kicked down to low until the bus got to speed. Something a PowerGlide transmission could do, even in the 1950s.

[speedracer1407]

It just might have been that the transmission kicked down to low until the bus got to speed. Something a PowerGlide transmission could do, even in the 1950s.

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.

[daerah]

For the most part, since the bus was traveling uphill, the electric drive-system wasn't really quite powerful enough to pull the hill at that speed, so the diesel-drive system (higher engine rpm's kicked on)...quite frankly, I LOVE it when it does this, that means I can get down the street with no problem.

The first batch of 4000s are SLOW, seemingly they rely on more battery-electric power and use less diesel power...with the exception of 4014 and 4029. Those two, to-date I've driven and they drive like regular 1000-series buses. I step on the gas, and the bus goes. No huge delay, no waiting to get up to 30mph by the way of the electric-drive system, then switching to diesel power...it just "get's-up-&-go" (as I say. lol)

The higher RPM (Diesel-power) is ideal, because we need to get up to speed QUICKLY (of that, like a regular diesel powered bus 1000 series) and be able to keep up with the schedule, especially when working on a line like the 156, where you're making EVERY forsaken stop, in traffic with short cycled stop-lights, where cars don't want to let us in, bikes who think they can out run us, joggers who dare us to hit them (but deep-down, we HAVE to slow or stop for them), or you've got the soccer-moms in tennis shoes just darting out into cross-walks with their puppies & kittens, while pushing their babies in the sports-stroller.

With all of those factors going, PLUS a slow take off of from an artic-4000, makes things for us, a little more sour...and ultimatelty for YOU all, 'the passengers' who are in rush to get home.

I hope cta speeds up ALL of the 4000s. The buses we received from North Park (P), are pretty fast...better than the Kedzie (K) originals.

[speedracer1407]

For the most part, since the bus was traveling uphill, the electric drive-system wasn't really quite powerful enough to pull the hill at that speed, so the diesel-drive system (higher engine rpm's kicked on)...quite frankly, I LOVE it when it does this, that means I can get down the street with no problem.

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.

The first batch of 4000s are SLOW, seemingly they rely on more battery-electric power and use less diesel power...with the exception of 4014 and 4029. Those two, to-date I've driven and they drive like regular 1000-series buses. I step on the gas, and the bus goes. No huge delay, no waiting to get up to 30mph by the way of the electric-drive system, then switching to diesel power...it just "get's-up-&-go" (as I say. lol)

I hope cta speeds up ALL of the 4000s. The buses we received from North Park (P), are pretty fast...better than the Kedzie (K) originals.

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.

[Busjack]

From the direction the thread is going, it appears that the electric assists don't do much to either (1) get the bus started or (2) prevent reliance on the diesel, especially on ramps. If that's the case, it appears that either it isn't working or it sure isn't saving fuel, like CTA claims. Normally, electric drive should supply almost instantaneous torque. You don't have gearshifts on the L, for instance.

[daerah]

A little off topic...the 800s were slow like the 4000s on the take off (acceleration) for some time. Until recently,..they've been tuned up, or (perhaps it worked itself out) to become faster. Most if not all of the 800s which I call baby 4000s are exceptionally fast...and to my sudden surprise, some of them,..like 802 and 807 arent governed at 55mph anymore...I hit 70...I couldnt go any faster, to see where it maxed out at...85 perhaps? But traffic wouldnt allow that. Ive heard of some 4000s aren't governed either...but I dont remember which numbers...but they were at North Park...but those have been shifted around from kedzie to 77th and back and forth, and possibly 103rd now...so who knows where those un-governed 4000s are.

[ryanbytes]

I like to sit in the back of the 4000s when I can so I can look out and down into cars because I'm nosy. I notice that before the bus accelerates it rolls back a little. It doesn't matter if it's an early delivery or late. In my experience they all tend to roll back a little before forward motion. It seems to go with the acceleration delay I'm reading about here.

[speedracer1407]

From the direction the thread is going, it appears that the electric assists don't do much to either (1) get the bus started or (2) prevent reliance on the diesel, especially on ramps. If that's the case, it appears that either it isn't working or it sure isn't saving fuel, like CTA claims. Normally, electric drive should supply almost instantaneous torque. You don't have gearshifts on the L, for instance.

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.