engine braking?

Question

i've read that gearing down and using engine braking saves fuel, compared with coasting up to traffic lights. But how can this be if the engine is in neutral it's not under load so it can't use much fuel. which one is right.

mr pink 'your not actually using any fuel as the gears are being driven by the wheels turning' but surely as you gear down and the revs pick you therefore use more fuel.

no i don't do racing downchanges but inevetably the revs will pick up as you gear down, unless you have a huge amount of roadspace. Sometimes you can't always have that amount of space in front.

mags, that answers pretty technical, but brakes or gears?

stirred a bit of a hornets nest up here!

Answer 1

Wow. There are a lot of people on here who assume they know how something works and also think they know exactly what they are talking about.

That's the problem with a lot of engineers I find, especially the new ones- they are almost scared to admit that they do not know something for certain.

What I'm curious about is if jake brakes (diesel engine braking) also save fuel, since disel engines are unthrottled. Maybe I'll ask the question some day.

Things to be concerned about with your question though...

Are you really going to notice the fuel savings in your pocket book? Doubtful.

Are your feel savings going to be greater than the added wear and tear on your clutch? I guess this depends at how good you are at shifting and if you double clutch.

Would you rather replace your clutch earlier or replace your brakes one more time?

I'm curious if the comments about the added wear and tear on engines are true or not. For the most part people just guess in cases like these. I wouldn't trust anything less than the results of a controlled test in an automotive test laboratory before I started telling people if there was wear or not but some may know better from mechanical experience. I would bet most are guessing.

Answer 2

Engine breaking does save fuel on a very small scale as when you coast you are using fuel to keep the engine ticking over, when you gear down your not actually using any fuel as the gears are being driven by the wheels turning! Just as the bump starting tecnique uses to kick start the engine turn over. The engine breaking comes as a result of the extra load of turning the engine and gear system.
**Yes the revs pick up but its not a guage telling you how much fuel's going in, its telling you how much the engine is turning over. They go up because you've changed the gear ratio and the rpm goes up due to the ease on the loading**
Also if you have a trip computer it reads your mpg according to your accellerator pedal not specifically your rpm. Its also more economical to drive at 30 in third gear than fourth gear as you don't need as much lead in your boot! Not massively-but it is!

Answer 3

Lets sort out fact from fiction. The only thing that slows a car or truck down when you remove the foot from the accelerator is the friction of the moving parts in the engine and compression ratio. Deisels have an apparent greater friction level because they have a higher compression ration on the pistons typically 20:1 where petrol cars are 8.5:1 typically. Using the gears to slow down happens because the reflected inertia of the vehicle that is presented to the engine is altered by the gear box (in a 2:1 gearbox the inertia is altered by the square of the ratio so the inertia is altered by 4:1). Why is this important because acceleration and deceleration are caused by torque divided by inertia. If the torque is the friction of the engine and the truck or car reflected inertia is presented to the engine with a higher gear ratio the in apparent inertia is lower and the the acceleration or deceleration is greater.

Answer 4

Down shifting uses more fuel, not less, because it increases the engine rpm. That is why it is an efficient braking method.
If you approach a stop in a high gear and shift into neutral just before stopping, your engine uses the least amount of fuel.

Answer 5

you are using fuel whether you use engine braking or not. Coasting (out of gear) is inadvisable because if your engine stops, you don't have the servo assist required for the brakes and you are likely to ram somebody. Also, to initiate a restart, you will have to turn the key fully off and this will engage the steering lock... so matey, your goose is cooked

Answer 6

In a modern fuel injected car, lifting off the gas completely shuts off the fuel supply to the engine, so that for the couple of hundred yards you travel in gear, you're using no fuel at all. If you put it in neutral, or dip the clutch, the engine uses a small amount of fuel to run at idle speed, so you use more fuel coasting.

Answer 7

Apparently, there is a lot of disinformation out there. First of all, if the engine is running, fuel is being burned in order to keep it running - whether fuel injected or carbureted. Second, engine braking only works on gasoline, spark ignition engines and it works because when you down shift into a lower gear, the rpm's increase but the engine is being throttled (assuming that you are not depressing the accelerator, the flow of air into the engine is being restricted). Thus, the amount of fuel that is being fed to the engine is reduced and the engine will attempt to decrease rpm's (if the vehicle is not heading down a steep incline). But, during engine braking, the amount of fuel that is being fed to the engine is greater than an engine at idle and no load.

Answer 8

Brake shoes/pads are cheaper to replace than the nowadays not so chunky gearboxes. I assume that because of the way you have couched your question you mean that you slide into a lower gear and then let the clutch up on a now ticking over engine, to slow down. Two things, you will burn your clutch out and secondly your vehicle will not be under safe control. Carrying out racing gear changes burns loads of fuel, is for the `half brains` and achieves nothing. Your vehicle should be in the correct gear for the speed you are travelling. As you slow down you should be able to slide into a lower gear as your speed decreases, using your throttle to match road speed with engine speed until you finally declutch when coming to a stop.

Answer 9

In a car or a small truck it is not as critical but the idea is the same.
You find in large trucks they have engine brakes which shut down half the cylinders to help slow down the truck and also using down shifting to help slow it down because it saves on brakes. I don't know so much about saving fuel but it will definitely help on the braking system.

Answer 10

Using the engine as a brake is correct. It does not save fuel like you say but it stops the brake pads and shoes wearing more rapidly. It is good driving to remain in gear, coasting can be dangerous especially if is wet or icy.