Theoretical engine efficiency query

Mr_Sukebe

Mr_Sukebe

Mr_Sukebe

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Here's a question that I've often wondered at, which I'm hoping some of the smarter among you can answer.

Imagine two scenarios, both of which start with exactly the same car, on exactly the same 1 mile long road section and the same conditions. No traffic. In each case, the car begins the test at 30mph. At the end of the road section, the cars both continue at the speed that they've accelerated to.

Scenario 1. Driver floors it, accelerates as fast as possible to 60mph, then drives at that speed exactly using cruise control

Scenario 2. Driver very gradually accelerates, such that at the end of the one mile stretch, he's now driving at 60mph.

Caveats:
- Clearly the driver will cover the 1 mile distance in scenario 1 faster. But that's not what this query is about
- Assume that we should ignore the additional drag factor for the fact that the car in scenario one will have a higher average speed. So please ignore the fuel implications for that

Taking all of the above into account, will scenario 1 use more, less, or the same level of fuel?
From a theory, the required energy to change to the velocity of the same mass should be the same, regardless of the time taken. So again, ignoring drag, is the fuel used the same?
 
Neither.

Best would be to floor it -until- the RPM in which the engine runs at its peak torque, keep it at those revs (use the gearbox to keep it as close to that torque peak as possible) and then when up to speed, put it in whatever gear enables you to maintain that speed with the minimum of throttle.

It really depends on the motor and where its peak torque is to say whether flooring or being gradual will be more efficient.
 
Wide open throttle runs a richer air fuel mix, so scenario 1 will use more to generate more power aswell as the higher average speed resulting in more roadload.
 
Scenario 2 will use way less fuel. I use scenario 2 style everyday, although I peak out at 50mph. The best way I have found is accelerate gently to the speed I wish to maintain and then maintain that speed. This gives the best balance of economy and speed.
 
not as straight forward as it first seems.... (as Hater said :p)

look up the efficiency of turbo formula 1 motors and where they are most efficient
 
and do you have a turbo f1 engine in your car? (would they be suitable for the mix of driving our cars require? how would they cope with a stop-start commute?)

I have to agree with Jonnycoupe - afaik, most cars have a closed and open loop feedback for their fueling, at normal throttle it uses closed loop to be as close to an idea air/fuel mix (stoich) as possible, but at full throttle it shifts to open loop where it uses a look up table and runs slightly richer than ideal (effectively putting more fuel than can be burnt). This change over point is linked to throttle position and, depending on the car, I think is about 70% (on a warm car - cold cars are always open loop).

Not to mention "booting it" in full throttle can lead to wheel spin (depending on car), which will reduce efficiency.

But, it's not as simple as accelerating very gently, as Hater said, you need to use the peak torque to efficiently accelerate to the desired speed (without breaking traction and probably ideally without flipping into open loop control, so under about 70% throttle position), before changing to probably the top gear.

i think.
 
thing is, in a vehicle it's not just the engine, it's the rest of the vehicle, and i'd say the external losses from drag etc are going be far more significant than any internal losses in the engine, so spending longer at the higher speed will use more fuel.

part of the logic of the trend of going with smaller engines with bigger turbos is that when driving efficiently you'll be at a wider throttle opening on a smaller engine, thus having less pumping loss, but still retaining the ability to generate high power if necessary. the main reason for this is because any loss reduction will be an economy improvement, and that there's a limit to how much they can reduce the drag on your average road car. of course they try to do both but it's incremental gains in both areas.

the problem i have with this concept is that you end up with a very binary engine- it's either gutless and effecient, or gutsy at the cost of still drinking fuel, and you lose the middle ground, which unless your a hypermiler living in holland you're going to need every time you meet a hill or perform an overtake.
 
Option 2 but then the average speed is higher so even ignoring the efficiency element the amount of effort is higher.

What would be an interesting take on it would be for scenario 1 to stay the same but scenario 2 sees the driver accelerating at a steady rate such that the cars cross the end point at the same time (i.e with the terminal speed of the second car at whatever it needed to be to catch up). Would be very car specific though I guess.
 
arfoll said:
Quite funny because option1 is really the way CPUs handle throttling these days - enter deep sleep states as quickly as possible. So ignoring drag/friction, you want to accelerate as fast as possible then coast in neutral with engine off :).
Click to expand...

Which is basically the strategy used in "Mileage marathon" contests (Popular back in the 70's, but seemingly not so much nowadays)
 
eviled said:
not as straight forward as it first seems.... (as Hater said :p)

look up the efficiency of turbo formula 1 motors and where they are most efficient
Click to expand...

They might be efficient for the fuel per power but the power is in massive excess of whats actually required for the mission description.
 
As above!
The BSFC chart is more or less the same for all automotive engines but there are small variations with the spec such as gasoline(throttled), Diesel(un-throttled), charge air, engine cylinder configuration.

FYI, downsizing is an example of a engine configuration ideology that forced drivers to use the optimal BSFC range. The smaller engine meant wider throttle open usage at higher revs (in normal road load conditions)with smaller capacity, turbo was added to compensate for the power loss.

There's a little bit more to it than just this but it explains the idea.
 
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