5

I'm driving an automatic, and often go down a big long hill.

Lowering the gear means I don't need to hold my foot on the brake the whole time, but does it burn up a lot of petrol?

8

Probably not. Engine braking works because the engine isn't generating power – on a gasoline engine it's soaking up power by pulling a vacuum in the intake as it draws air past the closed throttle. Since the engine is at idle it will be using minimal or even zero fuel as you coast down the hill.

Without knowing the specific car and engine your wondering about it's hard to do more than answer in general. But in the case of VW diesel engines, the claimed injection quantity actually does drop to zero when coasting down (but diesels don't engine brake in the same way that a gasoline engine does). Because there is no throttle, they don't pull a vacuum in the intake.

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  • Complete noob here—if the engine is idle, what's making all those revving sounds? – Mirror318 Sep 8 '16 at 21:29
  • @Mirror318 The engine is at idle power - the throttle is closed and the driver is asking for no power from the engine. But since it is being driven by the transmission it is turning over faster than idle. – dlu Sep 8 '16 at 23:46
  • The throttle is never completely closed. – 3Dave Mar 22 '19 at 17:42
4

For a gasoline engine, this depends a little bit on how smart the fuel injection system is, but in no case will it use any more fuel than normal for that RPM.

Most modern gasoline engines are smart enough to detect this condition, and simply cut fuel entirely, so the engine uses no fuel at all while engine braking.

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  • "fuel than normal for that RPM"—doesn't the RPM shoot up with engine braking though? So yes, it would burn a lot of petrol? – Mirror318 Sep 8 '16 at 21:51
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    @Mirror318 no higher than it would be if the engine was driving at that speed in that gear. that's absolute worst case. if you're spiking the rpm significantly, you're engine braking wrong. – Leliel Sep 8 '16 at 21:54
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    usual engine braking technique is to just let off the throttle. that restricts airflow to the engine, but the transmission forces the engine to keep rotating at the same revs, so it saps lots of power because there is lots of resistance to pulling in air past a closed throttle body. – Leliel Sep 8 '16 at 21:55
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    If you need more braking than the engine can provide, well, that's why there are actual brakes on your vehicle. – Leliel Sep 8 '16 at 21:57
  • If the catalytic converter is cold, fuel isn't cut. Also, on some cars that have air conditioning, if the compressor is running it would create extra drag, so to make the engine braking consistent with A/C on or off, some fuel is injected if A/C is on. At least in my 2011 Toyota Yaris fuel was injected if A/C was on. – juhist Feb 8 '17 at 16:14
-1

If you need to use the accelerator pedal because it is causing you to brake too much then yes it will consume more fuel.

However engine braking is very bad for the engine and transmission in the long run. You will wear out your piston rings, bearings, journals, cams, etc... and to top it off your transmission will be exposed to prolonged high heat (which is the #1 killer of transmissions).

So just use your brakes and replace them... engines and transmissions are not cheap to fix, brakes are. (Unless you are driving a mountain like pikes peak :rollseyes:)

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  • 1
    Trevor, this is really not true. Engine braking is recommended for most vehicles when on long downhill sections, and also when coasting to a halt as it is so much more gentle on the engine than other ways to stop. If you have a study that says otherwise, please post the link here. – Rory Alsop Jul 5 '19 at 13:34
  • Your engine/transmission coolant/fluid might be regulated to the thermostats temperature (180-210). The moving metal components (inside the combustion chamber, clutch packs, bands and drums, and planetary gears) are however not going to be the same temperature. Heat can only move away from the source so fast... Just because the idiot gauge reads 210 degrees Fahrenheit does not mean you aren't stressing the components forced to rotate faster than normal. Combustion temperatures exceed 1500 easily... Just put that into perspective. – Trevor Morgan Sep 6 '19 at 1:09
  • Remember Trevor, when engine braking, the engine is running cooler (far less combustion, if any) but coolant still moves heat. So your basic premise is incorrect. In any case, the engine is designed to effectively handle load, whether acceleration, speeds or even towing, where the heat generated is considerably higher. – Rory Alsop Sep 6 '19 at 7:54

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