Does Engine size and gear ratio REALLY matter?
 

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.

For P&G purposes, 2.0 + short gearing would probably be better. Drop into gear, load the engine and get up to glide speed, the shorter gearing will allow a brisker acceleration as will the bigger engine, therefore less fuel used. However, you are removing way to many equations from the scenario for the question to be really relevant anyway.

i have a 1.8 dohc 4 cyl and 4.11 gears. much power! p&g is the answer!

if I go from 0-40mph I will only shift to 3rd. my foot is over half way in it, clutch in kill switch in.

45-55mph p&g I will be in 4th for the pulse.

if I don't p&g I will get low mpg.

heavy foot low gear. the complete opposite from what it should be. It took awhile to figure that out.
I learned from a high speed p&g run. 60-75 I was shocked that I was getting the same mpg from the same run at 45-55 p&g.

now I want more power!!!

Both matter, think backwards, you want the smallest engine geared for the highest load at the lower left quadrant of that bsfc window at 100 KPH.

You are looking for overall gearing to give you peak bsfc rpm, say 1800 in your 1 liter engine at 100kph-61.7 MPH. It could be a 4 speed (direct drive 4th) with a 1.7 to 1 diff ratio. You want gears close enough to provide adequate acceleration.

a 6 speed with redline speeds of 22, 44, 65, 100, 180 mph would be very good, and some form of super-turbo-charging for short term overburst power.

Like the Fiesta ecoboost but 700 pounds lighter with half the aero drag.

regards
mech

In my experience you can extract good numbers out of short-geared, bigger engined cars, but it's a lot more work (constant shifting, etc). Efficient cars make this easier and more comfortable.

I think it matters.
The 3/4 ton suburban I just picked up with 454 engine, non-overdrive, non-lockup converter transmission and 4.10 gears gets 6.5mpg driving in town.
So what do you think?

All other things being equal, if you have a 1.0L engine and a 2.0L engine producing the same amount of power to go a specific speed with your car and both engines have the same BSFC at that vehicle speed and power output, they will get the same fuel economy at that moment.

Making that happen in real life is very difficult.

Now, to be perfectly clear: This is a theoretical, on paper idea. I realize that there's a billion other variables to this equation.

That said, the thought process here is, in real world ideology, to compare in regards to the idea of, say, an engine swap to a different engine or changing out transmission for different gear ratios, all while still using the rest of the car. Even then, there's obviously variables...

The other real thing to learn from this thought is that, with proper pulse and glide, does it really matter what your drive train is?

Take that suburban for example. If you were to swap in a smaller engine, would you really gain anything in terms of economy, assuming the smaller engine used equivalent technologies? Hard to account for an automatic transmission, since you can't apply the same P&G methods quite the same here.

Just an "on paper" discussion about what's under the hood, not so much the vehicle, tires, aerodynamics, etc.

read this thread

http://ecomodder.com/forum/showthrea...ive-30423.html

If you have a sufficiently small engine with sufficiently tall gearing, your pulses become infinitely long. In the case of the Insight, the 1.0 in 5th gear is running at very high loads just cruising, especially when it leans out to ~25:1 AFR.

A factor to consider in the hypothetical argument is, what RPM will these engines be running? Where is peak BSFC? If you're gearing to get the same power output, the 2L engine will be spinning half as fast, and although low RPM = less frictional loss, you'll rarely fine a gasoline engine in that displacement range that's terribly efficient at ~1000RPM due to (I think?) the geometries required by gasoline's flame speed.