Engine theroy, correct me

I'm sure this is too simple, so please correct my dumbness.

Will these two examples use the same amount of fuel?

Engine 1: 2.0 liter 4 cylinder @4000rpm

Engine 2: 4.0 liter V8 @ 2000rpm

Both have the same bore/stoke and valve size. They use the same amount of air (8,000 liters per minute) so at a perfect 14.7:1 A/F they would use the same amount of fuel, right?

Now which is making more power? The V8 is slower and has more internal friction but has more firing events per rotation. More rotating mass = more torque?

I would think so. Is there an ideal efficiency speed for the various gasses going into and out of the combustion chambers? I assume there must be but I don't know nearly enough about it.

Would the factor that the V-8 should be wearing at half the speed weigh it to the equation?

Don't forget volumetric efficiency. If engine 1 is a 3SGE BEAMS motor and is running at 99 or 102% efficiency, that's more air and more fuel than a 4L v8 that isn't that high. Although that BEAMS motor is probably in the high 80% at 4K RPM.

that is the thing. A smaller engine at WOT and Max RPMS is at it's most efficient in producing power for amount of fuel used.. a larger engine at half throttle and half the Max RPMs is going to less efficient.

It is about gas speed, volume, and internal friction and a matter of sheer mass.

It's 'Theory' not theroy.

You did ask to be corrected.

mad_machine wrote: that is the thing. A smaller engine at WOT and Max RPMS is at it's most efficient in producing power for amount of fuel used.. a larger engine at half throttle and half the Max RPMs is going to less efficient. It is about gas speed, volume, and internal friction and a matter of sheer mass.

I thought the faster an engine runs, the less efficient it gets, as the cylinders don't fill with air as completely each cycle. So in my mind, an engine at idle is using fuel more efficiently than at WOT, as the cylinders have more time each stroke to properly fill with air.

But besides that, and all other factors being constant (which they never will be in the real world), I would expect the assumptions stated in the original situation to be valid.

No, motors get more efficient, volumetric wise, at high RPM. a 4AGE 20valve, for example, does like 110% at 7K RPM from my personal tuning experience. Yes, it is taking in 10% more air than the pistons are displacing.

With more power events per revolution the v8 might have an edge, depending on how you were measuring or using the power. Similarly a piston moving twice as fast probably uses more than twice the power.

Of course the 4-cylinder would probably generate less heat, which would in theory mean greater efficiency, so who knows?

Let's throw in a middle contender. 3.0L, Six cylinder, 2667rpm

One can't make a real generalization of power and voumetric effiency. Some engines pump air really well at high RPM, others do it a lower RPM. Depends on the head, valves, combustion chamber, intake and exhayst manifolds, etc.

Now if one assumes that they DO flow the same amount of air, that constrains the argument some. On a potential energy standpoint, they are running the same if they are pumping the same air and fuel.

Next, enough info isn't really there to go much beyond there. The question is the extra friction of a V8 more or less than the spinning friction of the I4. If one says that at 2000 rpm, the losses are porpotional to the size, then it's safe to say that the I4 at 4000rpm will be higher than the V8 at 2000rpm, since friction RPM^2.

But friction varies enough that one really can't generalize the friction between two engines. Crank design, valvetrain design, springs, etc.

Then pumping losses. Again, a lot has to go into that. Not enough info.

Finally, combustion efficiency. And, again, there are too many variables to make a generalization. Some engines burn really well at low engine speeds, some do it really well at high speeds. Some at astronomical speeds, others at relative walking pace.

Just volume and speed can't really make many generalizations of engines. Too many factors.

Another thing to consider in your analogy (just nitpicking), the V8 theoretically has twice the breathing of the 4. You mentioned same valves... that means the V8 has twice as many of the same sized valves.

In your analogy, there are tons of variables that we can assume are somehow accounted for.

The V8 and I4 at those respective revs you list would probably make very similar torque at 2000 and 4000 rpms respectively, however the V8 will have the breathing capability to rev higher, therefore it has a much higher HP potential.

Ponder the old equation: HP = TQ x (RPM/5250) So, at 5250 rpms, all engines make equal torque to HP.

The thing that will cripple the V8 in your analogy is that it makes more friction at 2000 rpm than the I4 makes at 4000 rpm. Surface area of the pistons, rings, bearings, etc means it has more than double the friction surface than the I4.

The I4 would have to rev twice as fast to make as much torque as the V8

Gearheadotaku wrote: I'm sure this is to simple, so please correct my dumbness. Will these two examples use the same amount of fuel? Engine 1: 2.0 liter 4 cylinder @4000rpm Engine 2: 4.0 liter V8 @ 2000rpm Both have the same bore/stoke and valve size. They use the same amount of air (8,000 liters per minute) so at a perfect 14.7:1 A/F they would use the same amount of fuel, right? Now which is making more power? The V8 is slower and has more internal friction but has more firing events per rotation. More rotating mass = more torque?

In theory they make the same power and the V8 makes twice as much torque, because RPM is halved.

In practice, all bets are off because the 4-cylinder has more friction in the engine and in the trans, while the V8 will require stronger drivetrain components that may take more power to turn...

My gut feeling is more simple: The larger engine has twice as much internal surface area, so it will lose more heat to the cooling system. Less power, torque, and economy. The "maintenance drag" of the accessories/oil pump/water pump/etc will be lower on the V8 to offset this but probably not enough.

If it was a 4-liter four cylinder at the low RPM, or a 2 liter V8 at the high RPM, then it would be a more interesting discussion...

But spinning slower the V8 has a longer burn time and may extract a bit more power by burning more of the fuel it consumes.

bentwrench wrote: But spinning slower the V8 has a longer burn time and may extract a bit more power by burning more of the fuel it consumes.

That's not neccesarily better. Burning slower also shaves off the top corner of the PV, which loses efficiency,

That, and speed isn't the only thing that effects burn speed.

Lots and lots of variables in engines.

bentwrench wrote: But spinning slower the V8 has a longer burn time and may extract a bit more power by burning more of the fuel it consumes.

The fuel's all burnt in the 4 cylinder, too. At least, it should be, for normal values of cam timing and ignition timing and air/fuel ratio. Emissions before the cat is measured as a handful of parts per million HC, so anything that doesn't get burned isn't going to be much.

The slower engine will require less ignition advance for the same optimal cylinder pressure curve, though, which IS a net benefit. Less advance means less negative work done.

Knurled wrote:

bentwrench wrote: But spinning slower the V8 has a longer burn time and may extract a bit more power by burning more of the fuel it consumes.

The fuel's all burnt in the 4 cylinder, too. At least, it should be, for normal values of cam timing and ignition timing and air/fuel ratio. Emissions before the cat is measured as a handful of parts per million HC, so anything that doesn't get burned isn't going to be much.

For most of the engines I've tested, running stoich, engines are 3500-6000ppm of HC's, and 5000-10,000ppm of CO- which is also an indicator of lack of combustion.

Which is .35-.6% and .5-1% respectively.

But that's not a direct translation to loss of energy- as both are more indicators of partial combusion. Basically, as Knruled points out, it's not going to mean a whole lot.

The slower engine will require less ignition advance for the same optimal cylinder pressure curve, though, which IS a net benefit. Less advance means less negative work done.

Sorta, kinda. Again, it's really hard to make a generalization of advance and burning. It should be better, but many engines don't get going as quickly as others when spark initiates.

While I've never seen many examples of combustion really falling apart at 2000rpm, between 1300-1500rpm, I've seen a lot of engines where it gets terrible. Sometimes slow and misburns, and sometimes it will knock- all on the same chamber. Not really sure what the core phyisics are, but it's really odd.

All of these variables that I mention is why there many normalized parameters in combustion engines so that you can compare one to another, even with different sizes and shapes. The most commonly used is brake specific fuel consumption- which measures fuel used for the power delivered.

My point exactly is that a small bore long stroke motor has a better BSFC than a short stroke motor of the same displacement.

What we are really talking about here is time (the small bore long stroke just magnifies it) for all the fuel to burn completely and be converted to mechanical energy.

mad_machine wrote: It is about gas speed, volume, and internal friction and a matter of sheer mass.

This. Assuming everything is the same except for the # of cylinders and that the timing and cylinder design optimization is the same for the respective RPMs, the 4 cylinder will still be more efficient due to internal friction.

The problem is this is very much a theroy based question as every engine has an optimal RPM to operate at. What you want to look at is the Specific Consumption rate which is Lbs of fuel per BHP per Hour. Those curves answer your question.

bentwrench wrote: My point exactly is that a small bore long stroke motor has a better BSFC than a short stroke motor of the same displacement. What we are really talking about here is time (the small bore long stroke just magnifies it) for all the fuel to burn completely and be converted to mechanical energy.

Some do, some don't. I don't mean to be flippant, but it's not really safe to make that kind of generalziation. Realistically, what that does is change the speed where an engine has the best brake spec fuel consumption, and what power it makes it at.

In reply to Flight Service:

BSFC isn't always in lb/hp-hr, it's also in g/kw-hr. Which is more mass/work in general terms.

The graphs are really interesting when you plot the 3D BSFC on RPM vs. Power graphs. Especially noting that when driving, you are generally trying to maintain one engine speed for a set of power requirements.

In reply to alfadriver:

So if I say MPH and you say KMH we are talking about different things? No. Lbs can be converted to slugs (standard equivalent to g) with the gravitational constant so the conversion does not change the order of the curve.

My statement is just as valid in lb/hp-hr as it would be in g/kw-hr.

Remember this is a theoretical question about 4 cylinder consumption vs 8 cylinder consumption. This gives a nice answer as to what is being compared.

The science knowledge on this board never ceases to amaze me. Thank you all for your input.

In reply to Flight Service:

The problem is that you said that the reporting is lb/hp-hr, as if it's "the way". When it's more a measurement of fuel used/energy delivered.

It does give a good comparison, which I also pointed out before.

I just want to make sure we don't get to narrow in definitions, that's all.

Much like the whole power = tq*rpm/5252 thing- that ONLY works if power is HP, Torque is lb-ft, and it's revs/min. Not Newtons, not watts, or rads/sec. It's a result of the units used, not some kind of law.

And if one wanted to convert BSFC into actual efficiency, units really matter. Looking at the math, it's pretty easy to make BSFC into actual raw efficiency- since in it's most raw form, BSFC is energy in (fuel mass)/ energy out(work out).

Efficiency is really easy to compare one engine to another.

alfadriver wrote: Sorta, kinda. Again, it's really hard to make a generalization of advance and burning. It *should* be better, but many engines don't get going as quickly as others when spark initiates.

Isn't the intersection of theory and reality fun?

Your numbers re: pre-cat emissions are startling. That is a lot higher than I have seen on catless cars in good operating condition. I will admit that the equipment you have access to is probably just a wee bit more precise than what I've ever worked with

While I've never seen many examples of combustion really falling apart at 2000rpm, between 1300-1500rpm, I've seen a lot of engines where it gets terrible. Sometimes slow and misburns, and sometimes it will knock- all on the same chamber. Not really sure what the core phyisics are, but it's really odd.

Makes perfect sense to me. If you ignite it early but it burns kinda slowly, then you have a lot of endgases getting compressed and heated and plenty of time for them to autoignite before the piston falls away.

Knurled wrote: Isn't the intersection of theory and reality fun?

Even more fun when an engine designer uses generalizations, and the engine they come up with acts nothing like what they intended it to be.

People think that we've come to the end of this archaic motor, and the reality is that we are still working on a REAL phyisics model of what's going on. There is a lot of room for improvement the more we understand.