<travisgod@[EMAIL PROTECTED]
> wrote in message
news:99df07d3-3a26-4823-9024-f0a7e0a237e3@[EMAIL PROTECTED]
>> How much fuel does an engine use when you are going downhill with your
>> foot
>> off the throttle, using engine braking? I'd expect it to be about as
much
>> as
>> with the engine idling. Just because the engine is running faster and
>> there's a greater throughput of air due to the more frequent induction
>> strokes, why should that cause more fuel to be used, assuming you have
>> fuel
>> injection rather than a carburettor?
>
> It causes more fuel to be used because the engine compression retards
> your increase in speed, decreasing your roll out and requiring you to
> come onto the gas sooner.
>
> If you let gravity accelerate you in neutral you WILL find that many
> hills will do work for you that require significant throttle just to
> overcome the engine compression at the RPMs that your engine spins at
> at this speed.
>
> ****...just coast on a flat. See how far you roll, losing little
> speed. Then, try the same thing by lifting your foot off the
> accelerator.
>
>> Is the fuel consumption the same if you coast downhill with the engine
on
>> but the clutch pressed and/or the transmission in neutral?
>
> the fuel consumption is the same WHILE you are going downhill,
> HOWEVER, in SOME cases, overcoming engine compression requires
> ADDITIONAL fuel.
Ah, I see what you're saying: you're prepared to accelerate down the hill
to
a speed above what you were doing on the flat, and let that carry you part
of the way up the hill on the far side. Yes, I agree: coating in neutral
will involve less braking so you'll go further up the other side before
you
have to go back on the power.
But I was assuming that considerations such as speed limits and safety
(there may be a bend as the hill goes from downhill to uphill) require you
to restrict your speed with some form of braking. Now that braking can be
conventional disc brakes, engine braking or regenerative braking.
I was querying the statement that coasting downhill in gear uses more fuel
than doing it in neutral, and you've agreed with me that it doesn't *while
you are going downhill*.
I agree that regenerative braking is the best because the energy lost as
heat in brakes or engine compression is put back into the battery for
regenerative.
I'm surprised that the assertion about optimal engine speed resulting in
better fuel economy isn't true. Maybe you do get better economy, but this
is
ffset by the losses in converting mechanical (kinetic) energy to
electrical
energy in a generator and then back to mechanical in the motors.
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