A question about long-stroke or undersquare engines

[mender]

My copy is back in Manchester, I've recently moved house so I've not shifted all my engineering books yet. It should be pretty near the front though.

I haven't had chance to read back up on it yet, it's why I've not posted anything on this.

It took a while, I've been busy with many things but here it is.

Internal Combustion Engine Fundamentals by Heywood, page 824:

"Thus for well-designed engines, where the maximum values of mean effective pressure and piston speed are either flow limited (in naturally aspirated engines) or stress limited (in turbocharged engines), power is proportional to piston area and torque to displaced volume."

Reinforcing what I said earlier:

If the engine specs are the same other than the bore to stroke relationship, an oversquare engine will have the same torque as an undersquare engine.

The oversquare engine will have slightly less friction hp loss so it will produce slightly more net torque than an undersquare engine with the same displacement, again from Heywood, confirming that:

Oversquare engines have the advantage in both torque and power production.

Just in case anyone was still wondering.

 

[jack action]

It took a while, I've been busy with many things but here it is.

Internal Combustion Engine Fundamentals by Heywood, page 824:

"Thus for well-designed engines, where the maximum values of mean effective pressure and piston speed are either flow limited (in naturally aspirated engines) or stress limited (in turbocharged engines), power is proportional to piston area and torque to displaced volume."

Reinforcing what I said earlier:


The oversquare engine will have slightly less friction hp loss so it will produce slightly more net torque than an undersquare engine with the same displacement, again from Heywood, confirming that:


Just in case anyone was still wondering.

I agree with the citation from Heywood. But when you say «If the engine specs are the same», you have to specify what engine specs. Because if mean piston speed is considered (and it should), oversquare or undersquare, the friction losses are the same for both.

I don't understand why people are still using displacement as a viable yardstick to compare engines. Displacement is composed with 2 independent variables: the bore area and the stroke. Multiplying the bore area by 2 will increase power and torque by 2 as well. But multiplying the stroke by 2 will increase the torque by 2 but do nothing for the power. Why? Because the stroke is essentially a «gear ratio» where the linear force of the piston is converted to rotational torque of the crankshaft. Yes, you can double the torque by doubling the stroke, but you will have to halve the rpm as well; just like a gear set does.

Fundamentally, it goes as follow:

The power of the piston, with its linear motion, is Force X Velocity where the Force = Pressure X Bore area. That's it. Once you are there, the only thing you can do is transform that power between force and speed, in linear form or rotational form. To find the torque, all you do is an energy balance for one cycle where:

E_rotational = E_linear

crankshaft torque X angular displacement = piston force X linear displacement

The angular displacement per cycle is 1 rev for a 2-stroke and 2 revs for a 4-stroke. The linear displacement is the stroke length (because the piston force is defined as the mean effective pressure acting during the power stroke only).

In the end:

power = BMEP X bore area X mean piston speed / # of stroke per cycle

torque = BMEP X bore area X stroke / # of rev per cycle

Where it «just happens» that bore area X stroke is the engine displacement.

That being said, I know that bore/stroke ratio has an influence on the combustion chamber shape and probably also on some stress factor on the mechanical parts. That is in those fine details that a discussion on bore/stroke ratio becomes interesting (where it is BMEP and maximum mean piston speed that will be affected).

 

[mender]

I agree with the citation from Heywood. But when you say «If the engine specs are the same», you have to specify what engine specs. Because if mean piston speed is considered (and it should), oversquare or undersquare, the friction losses are the same for both.

That being said, I know that bore/stroke ratio has an influence on the combustion chamber shape and probably also on some stress factor on the mechanical parts. That is in those fine details that a discussion on bore/stroke ratio becomes interesting (where it is BMEP and maximum mean piston speed that will be affected).

You're preaching to the choir if your post is only directed at me. And you basically reiterating what Heywood stated.

And I did specifically specify the specifications here:
"The oversquare engine will have slightly less friction hp loss so it will produce slightly more net torque than an undersquare engine with the same displacement, again from Heywood, confirming that: "

 

[jack action]

You're preaching to the choir if your post is only directed at me. And you basically reiterating what Heywood stated.

And I did specifically specify the specifications here:
"The oversquare engine will have slightly less friction hp loss so it will produce slightly more net torque than an undersquare engine with the same displacement, again from Heywood, confirming that: "

I'm just thinking out loud, not trying to convert anyone in particular. I just think it's misleading to give so much importance to displacement when trying to determine engine performance.

Where I differ from Heywood is that it's not the bore/stroke ratio that gives the advantage, it's the increase in bore area. 2 engines with the same bore area, one with a bore/stroke ratio of 1:1 and the other of 2:1, will produce the same power output and are equivalent on my point of view. The fact that one has a displacement or a bore/stroke ratio twice as big as the other is irrelevant. Even if the smaller engine has less torque and a higher rpm, it's nothing a gear set with a gear ratio 2:1 won't cure.

Where does the «less friction hp» comes from for an oversquare engine? The only way I can think of is by keeping the same connecting rod length, which implies that the rod/stroke ratio would increase, reducing lateral forces. Is that what you mean? Because from what I know, everything being equal, only the mean piston speed should affect the friction losses.

 

[mender]

Where does the «less friction hp» comes from for an oversquare engine? The only way I can think of is by keeping the same connecting rod length, which implies that the rod/stroke ratio would increase, reducing lateral forces. Is that what you mean? Because from what I know, everything being equal, only the mean piston speed should affect the friction losses.

Since torque is proportional to displacement, it's only reasonable to compare the torque output of the engines using the same CFM passed through the engine and therefore the same rpm.

For a given displacement, the mean piston speed will be lower at any rpm for an oversquare engine but the gross torque output will be the same as the undersquare engine. Friction losses will be lower at the same rpm because the piston speed will be lower, meaning that the net torque output will be higher for the oversquare engine.

R/S ratio is assumed to be the same.

 

[jack action]

I see your point.

That's what I meant by «If the engine specs are the same»: I assumed the mean piston speed was constant, you assumed volumetric flow was constant.

On your point of view, you get a slight increase in torque and power because of the smaller friction losses;

On my point of view, I get exactly the same torque but a huge increase in power because of the increase in volumetric flow.

I enjoy looking at things through different eyes!