Does Engine size and gear ratio REALLY matter?

[aerohead]

Originally Posted by NoD~

Something I have been pondering lately...

Lets say you have a 1.0L engine VS a 2.0L engine. They both use the same technologies, such as fuel injection, cam specs, etc. Lets assume for this comparison that they weigh the same. Efficiency is just about even on these engines.

In another realm, we have different gearing! One is the "performance" gearing and one has long, tall gears for low RPM cruising. Again, weigh the same, efficiency is equal.

So here is the question: If one is to do nothing but P&G with Engine off... is there really going to be any difference in the end result of MPG?

Here's my thoughts on it: If they both consume an equal amount of fuel to produce an equal amount of power and things like idling is thrown out of the equation, should engine size actually matter? And with gearing: As long as you stay in the appropriate RPM range (such as referenced to a BSFC), is there really any point in gearing changes as long as you don't cruise in a set gear?

Of course, in the real world, a 1.0L will probably weigh less than a 2.0L and you will have to occasionally cruise in gear, but I wanted to throw out the variables here and compare to see if drivetrains are really going to make the difference when doing the extreme and P&G+Eoff all the time.

Was just curious on some thoughts on this matter.

If you're talking about pulse and glide with the engine off driving all the time,the real issue will be dodging all the bullets from irate and homicidal motorists who's mpg you're killing on city streets.Along with Earth,who might have to get a gun also.
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Academically:
*you'd have to specify a particular vehicle
*you'd have to specify a particular engine 'kill' velocity
*you'd have to specify a particular acceleration rate to reach your engine 'kill' velocity.
*The engine and gearing would be optimized for the highest BSFC under these specific conditions and they'd be optimized for only these conditions.
*if you drove any other way,performance would suffer
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*the larger engine would shrug off the acceleration ,operate at partial load,lower friction.
*the smaller engine would labor under acceleration,operate at a higher load fraction,higher rpm,higher internal friction.
*a small supercharged engine would become a 'larger' engine under boost for the acceleration phase.
*power IS power,but you'd have to analyze the efficiencies at which each powerplant developed the power band you sought.
*IC engines get their highest BSFC at a constant load,constant speed,rpm.They could see 33%thermal efficiency 'cruising',whereas 25% during transient loading.
*a recent Honda Accord with eAssist has no transmission.The engine operates only above 40-mph,and is directly coupled to the drive wheels right up to top speed (like many Bonneville racers [ 100% efficient power transfer])

*Pulse and glide would be the worst-case-scenario for an I.C. engine.

[cosmick]

Smaller engine = lighter and less torque, so trans / axle can be lighter, so suspension can be lighter, et cetera. IF mpg was the only thing, then smaller is better. But MPG can't matter until the car is fun enough that it doesn't make the driver suicidal.

[user removed]

My VX pulled fine from 1000 rpm in 5th gear @ 30 mph. Works out to 180 mph in 5th at 6k rpm. At higher speeds with significant aero improvements that ratio could go up to 40 MPH at 1000 rpm.

Maybe some here should try a 58 VW bug when they talk about underpowered vehicles, I think the 58 was 28 hp when speed limits were 70 here. I had no problem keeping up with traffic until the grades got above a couple percent. Afton mountain in my 63 (170 cu in 6 cyl, Valient was max in 2nd gear to 55 mph, then 3rd and speed dropping off until you went back to 2nd at 35-40, up to 55 then back to 3rd for several miles. ON the dowhill grade you worried about those 4 wheel drum brakes, no power assist and a single circuit system, meant when you boiled the fluid, you could easily die.

regards
mech

[oldtamiyaphile]

Originally Posted by Old Mechanic

Maybe some here should try a 58 VW bug when they talk about underpowered vehicles, I think the 58 was 28 hp when speed limits were 70 here. I had no problem keeping up with traffic until the grades got above a couple percent. Afton mountain in my 63 (170 cu in 6 cyl, Valient was max in 2nd gear to 55 mph, then 3rd and speed dropping off until you went back to 2nd at 35-40, up to 55 then back to 3rd for several miles. ON the dowhill grade you worried about those 4 wheel drum brakes, no power assist and a single circuit system, meant when you boiled the fluid, you could easily die.

I drove an Oval window once (I think that would make it 58ish), but it had cable brakes, so no chance of boiling the fluid

[P-hack]

I get better mileage in small 3cyl cars than with larger ones, with or without p&g.

The smaller engine is more likely to come with a smaller/lighter car.

[some_other_dave]

Originally Posted by NoD~

Lets say you have a 1.0L engine VS a 2.0L engine. They both use the same technologies, such as fuel injection, cam specs, etc. Lets assume for this comparison that they weigh the same. Efficiency is just about even on these engines.

What do you mean by "efficiency is just about even"? There are lots of different ways to measure or state efficiency...

A smaller-displacement engine will either have fewer cylinders, or will have smaller cylinders and/or shorter stroke. Those all change the efficiency and power-delivery characteristics of an engine. (Long-stroke motors tend to make more torque, short-stroke ones can rev more easily, for instance. And fewer larger cylinders tend to have less friction than a larger number of smaller ones.)

You can mess with all sorts of things to get similar fuel consumption numbers out of your 1.0 liter and 2.0 liter engines for some circumstances. But I don't think you can match the full BSFC curves.

If you do match consumption figures and/or BSFC, you've still got a different amount of maximum power available to you from those different-displacement motors. So 80% load on the larger one will be making more power than 80% load on the smaller one.

It gets complex pretty quickly...

Now, if you somehow matched fuel consumption on each engine completely, you'd have the same fuel consumption in each case. But that's a tautology--assume X to start, and you end with X.


In the real world, a smaller engine should still get better MPG than a larger one. Even doing P&G--but the pulses that the car with the smaller engine makes will be slower.

-soD

[Big Dave]

Both matter.

For diesels engine size is not as serious at a given engine RPM as it has no throttle. For throttled gas engines it matters considerably.

Gear matters big time as engine frictional HP varies with the cube of the engine speed (for a given displacement and road speed).

My pickup has an exotic drive train that gives me a net 1.94:1 gear ratio. I get 30% better MPG than stock and that's with my aero package currently compromised. I cruise at 1325 RPM @ 70 MPH.. The same truck stock cruises at 2000 RPM @ 70 MPH.

[Arragonis]

The simple answer it seems is "it depends".

[cosmick]

Originally Posted by aerohead

the larger engine would shrug off the acceleration ,operate at partial load,lower friction.
*the smaller engine would labor under acceleration,operate at a higher load fraction,higher rpm,higher internal friction.

I can't agree with this. The dynamic compression ratio DCR is highest at wide open throttle WOT, so that's when any given engine is getting the most energy from every drop of fuel. This is even more so at the RPM at which torque peaks at WOT, but then you'd need a microscopic engine to cruise at WOT at peak TQ without it making excess torque, then you'd have to run it at that road speed all the time, use nitrous for passing.
As for greater friction at WOT, this really isn't so if the oiling system has excellent design and is working as designed.
However, the slower the rate of acceleration, the less fuel used. Think how one man can get a car up to 5 MPH gradually, but you'd need 6 Clydesdale horses to do it quickly.

[cosmick]

Originally Posted by some_other_dave

In the real world, a smaller engine should still get better MPG than a larger one.-soD

Should, but often doesn't. Even overgearing the smaller engine, it's working at WOT a lot more of the time, and above 85% throttle, the A:F richens from 14.7:1 in closed loop to around 12:1 in open loop. Plus those extra revs from overgearing.
I can find you 1.1L cars that never could do 30 MPG, and I can find you 6.0L cars, with more drag, that can return 35 MPG consistently, no mods to either.
So what if the 1.1L is making more HP / Liter? That's only about RPM. The 6.0L meets ULEV standards, and puts down 5 times the power.