How small is too small for an engine?

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

Ford I do believe.

I remember seeing a handful more Dodge Rams with a V10 than any Ford other than the vans fitted with a V10, yet such engine layout was never so common here not even among those who could privately import such rides. On a sidenote, it's quite interesting that Ford ultimately replaced that SOHC 6.8L V10 with a pushrod 7.3L V8. While it may seem pointless to refer to the Ford V10 as small, it's interesting to look at how a seemingly "outdated" engine may even fare better.

[Isaac Zachary]

Originally Posted by ksa8907

Summary: engine size is nearly irrelevant for FE, so many other factors go into the equation.

I'm not so sure about that. Seeing how I seem to get better fuel mileage by pulse and gliding, which is essentially making the engine run at a higher load on purpose, even in a 4,000lb car with a little 2.5L 4 cylinder engine that at my altitude is producing around only 100hp max, I do believe that there's better fuel mileage to be had by sizing an engine so it runs at an even higher load for the average driver that isn't going to be pulse and gliding all over the place like me.

[ksa8907]

Originally Posted by Isaac Zachary

I'm not so sure about that. Seeing how I seem to get better fuel mileage by pulse and gliding, which is essentially making the engine run at a higher load on purpose, even in a 4,000lb car with a little 2.5L 4 cylinder engine that at my altitude is producing around only 100hp max, I do believe that there's better fuel mileage to be had by sizing an engine so it runs at an even higher load for the average driver that isn't going to be pulse and gliding all over the place like me.

So, to test your claim that smaller engines give better fuel economy. You would suggest that a 600cc motorcycle engine would return great FE numbers in real world use when installed in a minivan?

Also you would suggest that a 7000cc engine in a Corvette would get poor fuel economy in real world use?

Or maybe it's the other part of the claim... that there are many other variables. Vehicle size, aerodynamics, transmission, hybrid?, city or highway use, towing, tire/wheel size and selection, 2wd/4wd/awd, curb weight, etc.

[Isaac Zachary]

Originally Posted by ksa8907

So, to test your claim that smaller engines give better fuel economy. You would suggest that a 600cc motorcycle engine would return great FE numbers in real world use when installed in a minivan?

Also you would suggest that a 7000cc engine in a Corvette would get poor fuel economy in real world use?

Or maybe it's the other part of the claim... that there are many other variables. Vehicle size, aerodynamics, transmission, hybrid?, city or highway use, towing, tire/wheel size and selection, 2wd/4wd/awd, curb weight, etc.

There are many factors, but it seems easy for people just shrug off the ones they don't think are important. Lots of people feel SUV's are plenty aerodynamic, that hybrid's don't help that much and most people don't even think about the transmission. But as they say, every little bit helps.

Brake fuel consumption efficiency is something that can't be ignored. Engines get their best efficiency at a certain RPM and a certain load. Stray from that one point and effiency drops. You can't make an engine that's 40% efficient at any RPM/load combination. Even a change of a couple hundred RPM or 10% load can and will change efficiency, maybe not always by a huge percent, but it will change.

So how do you keep the RPM's and load as close to optimal as possible? There are many factors there too, as you said. What would a person buy a minivan with a 600cc engine for? Around town, to cruise down the highway, to climb steep mountain passes in? What kind of gearing would you put in it? Would you put 15 gears in a manual transmission so driver can keep the engine in an optimal range as much as possible? Or throw a 5 speed in there with gearing designed for as much acceleration as possible, keeping it geared really low?f

Note that many manufacturers have placed less powerful engines (what I meant by "small") in their CUV's than their sedans to try to get better fuel mileage in compensation for the worse aerodynamics. A lot of where hybrid's get their better fuel efficiency isn't from regenerative braking, but by keeping the engine smaller (or less powerful) so it will run closer to optimal efficiency loads, but with an electric motor to assist where the engine runs out of power or is too overpowering to be efficient.

[ksa8907]

Originally Posted by Isaac Zachary

There are many factors, but it seems easy for people just shrug off the ones they don't think are important. Lots of people feel SUV's are plenty aerodynamic, that hybrid's don't help that much and most people don't even think about the transmission. But as they say, every little bit helps.

Brake fuel consumption efficiency is something that can't be ignored. Engines get their best efficiency at a certain RPM and a certain load. Stray from that one point and effiency drops. You can't make an engine that's 40% efficient at any RPM/load combination. Even a change of a couple hundred RPM or 10% load can and will change efficiency, maybe not always by a huge percent, but it will change.

So how do you keep the RPM's and load as close to optimal as possible? There are many factors there too, as you said. What would a person buy a minivan with a 600cc engine for? Around town, to cruise down the highway, to climb steep mountain passes in? What kind of gearing would you put in it? Would you put 15 gears in a manual transmission so driver can keep the engine in an optimal range as much as possible? Or throw a 5 speed in there with gearing designed for as much acceleration as possible, keeping it geared really low?f

Note that many manufacturers have placed less powerful engines (what I meant by "small") in their CUV's than their sedans to try to get better fuel mileage in compensation for the worse aerodynamics. A lot of where hybrid's get their better fuel efficiency isn't from regenerative braking, but by keeping the engine smaller (or less powerful) so it will run closer to optimal efficiency loads, but with an electric motor to assist where the engine runs out of power or is too overpowering to be efficient.

I agree, engine size is nearly irrelevant because so many other factors play into it. The engine and gearing is matched to the vehicle and intended use.

[redpoint5]

Difficult to go much smaller

[Isaac Zachary]

Originally Posted by ksa8907

I agree, engine size is nearly irrelevant because so many other factors play into it. The engine and gearing is matched to the vehicle and intended use.

I never said that engine size is nearly irrelevant because it isn't. Take your 7L sports car you mentioned. How on earth do you gear it so it runs at close to the most efficient load as you cruise along the highway? You can't, it's impossible, unless we're talking about 100mph highways. Will the car still get better fuel mileage than the large infenstation of SUV's and CUV's because it's going to be more areodynamic. But show me a 7L sports car that gets as good of mileage as a Prius.

[some_other_dave]

I don't think you can really say that engine size is irrelevant. I mean, if we want to pick absurd examples, how about a diesel ship's engine? It's only a few hundred tons, after all...

Smaller engines will have less mass to move, generally have fewer moving parts, generally have less coolant and oil to heat up, definitely have less engine to heat up, should have less internal friction, and so on. Calling all of that irrelevant is ignoring something that shouldn't be.

Remember that cars built for top MPG contests have little tiny motors. If engine size were truly irrelevant, at least some of the winners would have stonking big V8s.

Note that I am not saying that engine size is the only thing--heck, there's a good chance it's far down the list of what is important for fuel economy. But my contention is that it is on the list. "Irrelevant" says that it isn't on the list at all.

Confounding a simple analysis is the fact that different vehicles and use cases have different engine requirements. A 660cc engine in a minivan might very well work for puttering around in the city with a load of groceries in the back, but it's not going to cut it going up a mountain on the freeway. And, of course, the fact that most consumers want MOAR POWAH (so they can get their groceries to 60 MPH in less than five seconds?) which is easier to do with a bigger engine.

I agree with the analysis in the first post, that the gearing is one important cause of smaller engines not getting the kind of fuel economy numbers that we might expect. But, if my priority is to get good fuel economy, I will choose the smallest available engine option and try to find better gearing.

[Isaac Zachary]

Here's a brake fuel consumption graph for a typical gasoline engine. Going up is increasing in Load up to full throttle at the red line and going right is increasing RPM. The green line shows the same power at different gear ratios, with two gear ratios points. The numbers are grams of fuel used to produce a kWh. The most efficient this engine can be is 206 grams of fue per kWh, or about 46.4% efficient if the fuel is your typical gasoline with about a kWh of energy for every 95.5g. Presumeably the "constant power output" line refers to the power needed to cruise at a constant highway speed on flat ground.

Now note the two gears (white dots along the green line), one at about 235g/kWh (40.6% efficent) and and the other at about 255g/kWh (37.5% efficient). Of course this vehicle could have one more gear at around 217.5g/kWh (43.9%). But if the engine were much more bigger and powerful the green line would be much lower, meaning the most efficient gear would be less efficient. A smaller engine, on the other hand, would raise that green line making it more possible to make a gear that sits closer to the 206g/kWh (46.4%) point.

Of course so far we've been talking about cruising at a steady speed. Once you need more power the green line goes up. (And if you need less it goes down of course). At the point of most power (high load and RPMs at top right corner where the red line on graph ends) you'll be using around 255g/kWh (37.5% efficiency).

Looking at the two dots representing the gears for this particular vehicle, if you were to need more power the lower dot would get best efficiency at WOT, around 220g/kWh (43.4%). The top dot looks like it could hit 210g/kWh (45.4%) at some 85% throttle, but could drop down to 215g/kWh (44.4%) at WOT.

Now what does all this mean for fuel mileage? Well, let's say this particular vehicle is getting 30mpg at the top gear in the graph. If it had one more gear at the right point you could cruise at the same speed and get 32.4mpg. But drop in a smaller engine and with the right gear that puts it around 210 to 215g/kWh you could be getting 32.7mpg to 33.5mpg.

However, the bigger and more powerful the engine the lower the potential fuel mileage, even with the best gearing. If the green line hits 235g/kWh and no better, then you maximum fuel mileage would be 30mpg at that speed.

Then comes into question if you need to cruise slower or faster. Going slower will lower the line, but reduce aerodynamic drag. So having a bigger engine with throw the line down into the even worse fuel efficiency zones making you potentiall get worse fuel mileage the slower you go. Having a smaller engine will mean your line will still be generally higher making it easier to put it into a gear that gets you superb fuel mileage, making you get better mileage over all due to still being efficient at the engine and getting less aerodyamic drag at the same time.

But going faster not only makes the line go up, it also makes the shift points go right towards higher RPM's. For an example, if you take the gearing as is in this graph and move the line up, the top gear there will possible cross over the 230g/kWh line, improving engine efficiency, but at the same time the greater aerodynamic drag would likely reduce fuel mileage. You can kind of see what would happen if the engine were larger or smaller or if there were taller gears on this same vehicle.

[cRiPpLe_rOoStEr]

Originally Posted by ksa8907

You would suggest that a 600cc motorcycle engine would return great FE numbers in real world use when installed in a minivan?

Depending on how mini it is, there are some motorcycle engines which could be OK. Even a 250cc single could fare well into some JDM kei microvan or their derivatives made under license somewhere else.

Or maybe it's the other part of the claim... that there are many other variables. Vehicle size, aerodynamics, transmission, hybrid?, city or highway use, towing, tire/wheel size and selection, 2wd/4wd/awd, curb weight, etc.

Sometimes I lurk about something as ancient as the Ford Model T engine being eventually suitable to other vehicles much newer than what it was intended to fit, naturally considering some improvements which used to be applied to it during the early hot-rodding days to achieve a performance level more suitable to what are now considered normal driving conditions.