If you’re cruising along at 1400 RPM and then you need to pass a car, you press on the gas and it goes to 4000 RPM. You can hear the engine revving up but it takes a second before the car actually accelerates.
What happens to the extra energy the engine produces during this second when the engine is producing more power but the car isn’t going any faster?
This doesn’t match my experience with a conventional automatic-transmission car. If I’m in a higher gear with lower engine RPM, and I press the accelerator down, here’s the sequence of events I get:
Are you sure about the sequence of events you’re reporting? Have you tried downshifting first to get the engine in a higher gear, and then putting the pedal down? If you do that, is there any sort of lag between your pedal-push and your car’s acceleration?
Note that the situation may be quite different if you’re driving any sort of hybrid vehicle.
IANA mechanic, but I would reckon the electronics are determining what gear to shift the transmission into, so as not to stall and also not to blow-up the engine.
I don’t know if it’s still the case, but at least in older cars, they didn’t have to decide. If you pushed the pedal all the way to the floor the transmission would automatically downshift. IIRC a mechanical linkage* took care of this. I would assume modern cars would use the computer to accomplish the same thing.
(*ETA Kickdown cable is the term I was looking for)
But my WAG would be that the output side of the torque converter needs time to get up to speed.
Yep, the trans has to kickdown to the passing gear.
AIUI, in a torque converter, the impeller is hard-coupled to the engine’s flywheel and the turbine (the thing impelled by the impeller) is hard-coupled to the transmission’s input shaft. IOW, the only way the turbine is going to spin faster is if the transmission downshifts, or if the car accelerates. OP is describing a situation in which the downshift has already happened (“you can hear the engine revving up”), so the turbine has already spun up to a speed commensurate with the newly-selected gear.
But since the two sides of the torque converter aren’t hard coupled to each other, they’re able to spin at different speeds. It’s why you can sit at a dead stop with the car in gear without stalling the engine. More modern ones will ‘lock up’ under the right conditions to avoid losses, but I don’t think that would apply here.
In the old days, you had manual transmissions and automatic transmissions. Manual transmissions had a clutch. When you stopped, you could push in the clutch pedal, and the clutch plate moved back and disconnected the engine from the transmission. You let the pedal out slowly, and the clutch would slip and grab harder and harder as you continued to release the pedal, until the clutch plate was firmly engaged and you had a direct, solid link between the engine and the transmission. Whatever speed the engine was going (RPM), multiply that by the gear ratio in the transmission for whatever gear you were in, and that’s how fast the wheels were spinning. There was no slippage, so it was mechanically very efficient.
Automatic transmissions needed some way to disconnect the engine and the wheels so that the engine wouldn’t stall every time that you stopped. There’s no clutch pedal in an automatic transmission (we’ll leave discussions about automatic clutch transmissions for another day… ahem…) so they used what is called a torque converter instead. You can picture a torque converter as two sets of fan blades that face each other, surrounded by a thick liquid. One “fan blade” (yes I know that’s not the correct term, but it’s descriptive) is connected to the engine, and the other to the wheels. When you stop, the fan blade connected to the wheels stops, but because of the liquid between the fan blades, the engine can still spin. All you end up doing is sloshing around the fluid. No biggie.
But here is the problem. When you make the engine go faster, that makes its fan blade spin faster, which sloshes the fluid around faster, which puts more pressure on the wheels to turn, and off you go. But you never have a solid connection between the engine and the wheels. There is always this fluid sloshing around in between. So you lose a lot of energy to sloshing fluid around.
This is why manual transmissions always gave you better gas mileage and efficiency in the old days.
Modern cars fix this issue by having what is called a lock-up torque converter. This functions like an ordinary torque converter when you stop. Fluid sloshes around and lets the wheels stop but the engine can keep going, so the engine doesn’t stall. But once you get going and are cruising down the road, the torque converter locks the two fan blades together and makes a solid connection between the engine and the wheels, so you no longer waste energy sloshing fluid around.
When you stop, the torque converter unlocks, and you slosh fluid around again so that you can stop again without stalling.
So what does all of this have to do with the OP?
Well, if you are driving a really old automatic car, the engine will rev up, but the car won’t immediately accelerate. This is because you are sloshing fluid around and wasting energy. Eventually the extra pressure from the fluid sloshing around faster makes the wheels turn faster and the car accelerates.
But if you have a modern car with a modern lock-up torque converter, and you GENTLY press on the gas pedal, the engine and the car will speed up at the same rate, because the torque converter is still locked.
However, if you’re not so gentle, the engine computer will tell the transmission that it needs to shift, the transmission will shift down into a lower gear, and the torque converter will unlock to allow some slippage between the engine and the wheels until the car gets up to speed. The engine RPMs go up much more quickly, but the car can’t accelerate that fast so you’re sloshing around fluid in the torque converter, but you don’t have a sudden thump and jerk like you would get if you were in a manual car and you shifted gears and just immediately popped the clutch out. Once things stabilize, the torque converter locks up again, because you don’t want to waste energy sloshing fluid around, and then at that point you are back to having a solid connection between the engine and the wheels, and your mechanical efficiency is good again.
So that is where your lag comes from. The torque converter unlocks, the transmission shifts, there’s some lag due to the mechanical inefficiency of transferring power through the liquid coupling of the torque converter, then when things settle the torque converter locks back up and you’re all good.
This is what a torque converter looks like inside:
The lockup mechanism makes a modern torque converter a bit more complicated (actual designs vary, this is just the first decent image I could find):
I would just like to say that that is a fantastically simple and very accurate description. Automatic transmissions are witchcraft, and in spite of the fact that I hate them and their existence allows people who have no business whatsoever behind a steering wheel to drive a car, the people who invented and developed them are absolute geniuses.
That is an awesome clear explanation, this site doesn’t need like buttons, except for that post.
Also, this wacky part is involved. It obviously couldn’t serve any purpose so I guess just a sculpture for the mechanic to admire. I bet they’d make good boot scrapers.
Others have given some very good explanations, but I’ll simplify it a bit. The transmission has shifted to a lower gear, and the second or two before the car accelerates is the time taken to shift to the lower gear.
Let’s say your cruising at 50MPH in top gear at 1400 RPM, you want to go faster so you press the gas hard. The transmission responds by shifting down two gears, and the engine responds by increasing RPM. You’re now going 50MPH in 2nd gear (or whatever) at 4000RPM. Except now the lower gear gives you mechanical advantage, and the engine makes more power at high RPM, so you can accelerate. The car will speed up as the engine RPM continues to increase.
And “modern” can mean anything made in the last 40 years, as they’ve been common since the 80s.
This is the interesting part to me - what makes the transmission respond when you press hard on the gas pedal? Is a computer making those decisions (which gear to move into), or something mechanical?
Excellent explanation, thanks. I was not aware of “lock-up” torque converters, which I suppose is one major reason modern automatic transmissions are more efficient than older ones. But also, earlier automatics had limited gear ratios – I believe the earliest ones were just two-speed. Today they’re usually four-speed or three-speed with overdrive, and many have 6 speeds or more.
For a very long time it’s been controlled by a Throttle Position Sensor and other input like speed sensors which these days are likely further processed by a computer but in the past used simpler electronic or electromechanical processing to control shifting.
A bit rushed right now, but I’m sure someone can fill in more details of the process.
In order to select the most appropriate gear at any given moment, the transmission needs to “know”, at a minimum, how fast the engine is spinning and how much torque (load) it’s making. before computers became part of vehicle drivetrains, intake manifold vacuum was the signal that served as a proxy for engine load: less vacuum in the intake manifold meant the throttle was more open and the engine was making more load. @Joey_P upthread also made reference to a “kickdown cable”. Based on this article, it sounds like kickdown cables weren’t a universal thing - and where they were used, they may have only come into play when you pressed the accelerator pedal all the way to the way to the floor.
Now that computers are running the whole show, they have direct access to the gas pedal position sensor and/or the throttle position sensor, as well as engine and vehicle speed sensors. They can take all of this information (along with knowing what gear the transmission is currently in) and go to a lookup table in the software to see whether to change gears, and if so, what gear to change to. That lookup table will have been populated by the vehicle manufacturer during drivetrain development work; generally speaking, when you’re cruising at a steady speed it will select the gear that provides the best possible fuel economy without lugging the engine (“lugging” = operating the engine at very high load and very low RPM, which can shorten its life).