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Old 05-09-2012, 07:08 PM   #1 (permalink)
lbeew
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comments on: Tip 44 of the 100+ hypermiling tips (efficient methods of slowing down)

Quote:
44) The most efficient way to slow down

When you *have* to slow down, here's an approximate heirarchy of methods, from best to worst.

1) coasting in neutral, engine off (ie. roll to a stop);
2) coasting in neutral, engine idling;
3) regenerative coasting (hybrid vehicles)
4) regenerative braking (hybrid vehicles)
5) coasting in "deceleration fuel cut-off" mode (in gear, above a certain engine RPM)
6) conventional friction braking (non-hybrid or hybrid)
Hello everyone, my name is lbeew and I am new on the ecomodder forum. This will be my first forum post aswell.

I was reading the 100+ Hypermiling tips and when I came to number 44, something sounded counterintuitive. In modern fuel-injected vehicles, as soon as you release the throttle, the engine management stops feeding fuel to the injectors, as long as the rpms are high enough (I am guessing higher than 800-1000 rpm). This means that when you release the gas and coast IN GEAR, the wheels turn your engine, without the engine management injecting ANY fuel. Thus effectively not using any fuel. (I think this is point 5 on the list...) So this is more efficient than point 2 on the list, where you put the gearbox in neutral, the wheels aren't driving the engine and so the engine is injecting fuel to keep it idling (around ~800 usually).

On a second note: points 3 and 4 imply regenerative braking, thus winning energy back from the motion of the car. This would actually produce energy, so these two options should be the best options, way ahead of coasting with engine off and certainly better than using fuel (in neutral) and coasting.

I would suggest the order of best techniques be:

1 / 2. regenerative coasting / braking
3. coasting in "deceleration fuel cut-off"
4. coasting in neutral with engine off (Can be bad for automatic transmissions! Not so bad for manual transmissions)
5. conventional braking with car in gear
6. conventional braking with car in neutral

I put point 3 above point 4 because with the car in gear you use virtually NO fuel in fuel-injected cars and you have A LOT more control over the car than when it is in neutral. Also better for downhills. And the engine braking helps in slowing down, while putting it in neutral slows you down less fast. And the point was to slow down.

Please let me know what you guys think about this.

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