Why a technology like freewheeling not used in cars to save fuel? We all know that when we move to neutral gear (coasting), the car travels farther than when it is in gear because there is no engine braking effect. I understand there is a safety issue there - but is that the only reason for not using it? What I am trying to say is - when the foot is off the gas pedal, stop engine braking effect using some technology to achieve better mileage.
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2$\begingroup$ Where did you read that it's not used in cars? When I am on the highway and take the foot off the pedal in my car, the engine idles and I am coasting (it would even turn off the engine with start-stop system, but the battery is too weak lol). $\endgroup$– OpticalResonatorJan 21, 2021 at 12:40
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$\begingroup$ Saab had a freewheel system for exactly this coasting... $\endgroup$– Solar MikeJan 21, 2021 at 12:42
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$\begingroup$ For example, DSG of VW has coasting function. When you release the gas pedal, the car starts coasting untill driver pushes brake pedal. $\endgroup$– ReactionicJan 21, 2021 at 13:16
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$\begingroup$ I missed a point - I was referring to manual transmission. Thanks for your answers. $\endgroup$– SRajJan 21, 2021 at 16:11
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$\begingroup$ Check your fuel consumption meter. Mine reads zero when coasting in gear as the momentum of the car is driving the engine without fuel intake. Disengaging the transmission means that the engine requires fuel to avoid it stopping. $\endgroup$– TransistorJan 21, 2021 at 17:48
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3 Answers
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If you are going down a hill, I believe coasting actually uses more fuel. The reason is that with modern fuel injection, leaving the engine in gear allows the engine to keep running without fuel being injected where as if you put the transmission in neutral (or use the clutch) the engine needs fuel to keep running at idle speed. On top of that is there is wear on the brakes and potential for brake overheating so coasting down a long hill isn't a good idea.
In any case, with my automatic transmission car, if I approach a red light with my foot off the gas and brakes, the engine seems to drop to idle speed so I'm guessing it is pretty much coasting.
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I assume because added complexity of the already complex transmission and you don't spend that much time actually coasting when driving because you are usually trying to maintain a constant speed which necessitates offsetting the frictional losses.
The only time I coast is down hills when I am already accelerating due to gravity more than I want to, or towards red lights where ultimately have to hit the brakes to stop anyways. Because if you did coast until come to zero right at the light with nearly no braking, it means you started coasting so far in advance that you held up everyone behind you. That second case might be tautological though since you would slow down less with engine braking and hold people up less.
The other time you might coast is when you are way above the speed limit and need to slow down but that isn't really something that happens often either.
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It overloads brakes. The original Buick dynaflo auto trans had essentially free wheeling ( many decades ago) . They needed new drum brakes twice per year, depending on miles driven. Buick re-engineered the Dynaflo in a year or two to give engine braking.
answered Jan 21, 2021 at 21:37