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Does a car still burn gas while moving with its own momentum?
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Bullhonkie
Golden Member
Yes of course. You wouldn't be able to make a blanket statement and say all cars exhibit this behavior. But I would say most relatively modern cars sold in the US from the past decade or so do. Or as Priit posted earlier in this thread:
LTC8K6
Lifer
In modern cars, the ECU recognizes engine braking situations and cuts off the fuel. The engine keeps turning due to the cars forward momentum.
It's the same as if you put the car in gear to push start it. The wheels turn the tranny, which turns the engine, which allows you to start it without using the battery and starter motor.
If you push a car with a manual tranny while it is in gear, the engine will turn. The forward momentum when engine braking is the same thing. It's as if the car is being pushed while it's in gear, except the mass and momentun is doing the pushing.
If you put the tranny in neutral, the engine will just idle like it would if you were parked.
It is the same for automatics, except you only get engine braking at higher forward speeds due to the torque converter. Below a certain speed, the converter will no longer transfer the forward motion back to the engine because there is no direct connection and the converter's speed gets too low. At that point, the engine just idles as normal.
It's the same as if you put the car in gear to push start it. The wheels turn the tranny, which turns the engine, which allows you to start it without using the battery and starter motor.
If you push a car with a manual tranny while it is in gear, the engine will turn. The forward momentum when engine braking is the same thing. It's as if the car is being pushed while it's in gear, except the mass and momentun is doing the pushing.
If you put the tranny in neutral, the engine will just idle like it would if you were parked.
It is the same for automatics, except you only get engine braking at higher forward speeds due to the torque converter. Below a certain speed, the converter will no longer transfer the forward motion back to the engine because there is no direct connection and the converter's speed gets too low. At that point, the engine just idles as normal.
the brakes will work well for about one good hard brake and then they won't work nearly as well (the power steering as well I believe). I wouldn't recommend this shutting the car off while driving around. the only time i do it is when coasting to a stop in the driveway.
Thegonagle
Diamond Member
You're correct, of course. But some people here obviously do need to see it with their own eyes before they'll be convinced.
I Saw OJ
Diamond Member
Incorrect, for the engine to work as an air pump and suck air through the intake and out the exhaust there needs to be combustion. Remember the 4 cycles.
1. Intake - The piston is pulled down drawing air and fuel into the combustion chamber
2. Compression - The piston moves up compressing the air and fuel mixture
3. Ignition - The compressed air and fuel is ignited by the spark plug pushing the piston down.
4. Exhaust - The piston moves up pushing out the exhaust gases.
Seth
greenman100
Banned
ok lets think this through.
gas ---> energy
momentum ---> energy.
if leaving the vehicle in gear requires more gasoline, AND converts momentum to kinetic energy, then where does all that energy go?
on newer cars, like since 1993 at least, the injectors shut off when the throttle plate is closed and engine RPMS are >~1500.
Get it right please 🙂
Yep, turning off ignition when driving is bad idea. Besides loosing power steering and brake booster, you also may lock the steering wheel. It isn't good for automatic gearbox, either. My friend tried it once when driving about 60mph, he almost crashed he's car when steering lock went on 🙂
Thegonagle
Diamond Member
Think again, Sethie!
When kinetic energy from the car's forward momentum is transferred through the wheels and transmission to the engine, there does not need to be ignition to move the piston down. (Kinetic energy pulls the piston down, as opposed to a chemical explosion pushing it down.)
LTC8K6
Lifer
The engine is always an air pump, whether it is running or being turned by push starting or engine braking.
The same amount of air will come out of the exhaust during engine braking, except that it will have almost none of the products of combustion in it.
Air is pushed into the engine because the pressure in the cylinder is lower than atmospheric pressure when the piston moves downwards. Air is pushed out of the engine by the piston as it moves upwards. This happens whenever the engine is turned, either by running or by engine braking. You will always have air coming out of the exhaust if the engine is turning and it is in proper working order.
Forced air induction is different of course. 🙂
The same amount of air will come out of the exhaust during engine braking, except that it will have almost none of the products of combustion in it.
Air is pushed into the engine because the pressure in the cylinder is lower than atmospheric pressure when the piston moves downwards. Air is pushed out of the engine by the piston as it moves upwards. This happens whenever the engine is turned, either by running or by engine braking. You will always have air coming out of the exhaust if the engine is turning and it is in proper working order.
Forced air induction is different of course. 🙂
greenman100
Banned
grrrr......
what happens if there's no fuel in the cylinder? the piston still moves up and completes all 4 cycles!!
I guarantee that the fuel injectors shut of fon coasting in a manual
Funny, when I've run out of gas on the interstate, the car didn't just stop, it coasted down, just like i'd taken my foot off the gas. ironic.
The truth is, I didnt know myself until a couple of weeks ago that the fuel is completely cut off when coasting in gear above idle. But when it was told to me, I realized it had to make perfect sense. So I totally understand the confusion but for the last god damn time:
If you are in gear, on a relatively modern car, your foot is not on the accelerator, and your engine is above idle speed, your injectors shut off and the car is not using any gas!
This is not an opinion, or a guess, this is a FACT.
MikeyIs4Dcats
Lifer
wanna buy a bridge???
thomsbrain
Lifer
this is only half true, because it only applies to new cars, which do shut off fuel flow when the car is coasting. the engine stops firing, but it is still turning and giving compression braking.
my 91 accord will happily do 30 MPH in 4th gear... with my foot OFF the gas! the car is set up to idle properly, in case you're wondering. so older cars, even fuel injected ones, do still send that minimum amount of gas.
I would guess that anything past 1995 would cut it off for sure, but I suppose to be absolutely positive youd have to check your own car for sure. Although Id say its a safe guess that the vast, vast majority of cars on the road today shut off fuel on engine breaking, making a blanket statement acceptable.
LTC8K6
Lifer
Once it drops down close to idle speed, yes. But during the engine braking period, when the engine is revving up to slow the car down, probably not.
Remember too that unburned fuel in the cylinder is a big no-no.
No, it would only be during engine braking, not just because you stepped on the brake pedal.
The reason you use a lower gear to save your brakes when coming down a mountain is because the engine is hard to turn. The compression of the engine prevents your vehicle from rolling too fast. This proves that the wheels do turn the engine, and that the engine is still compressing and pumping air. If it weren't, you wouldn't get any engine braking.
It works particularly well on high compression engines like diesels.
Thegonagle
Diamond Member
I've also heard that most "motors," with throttle input, will also fire on every fourth cycle. 😉
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