So, what is it that makes an engine accelerate?

[steelcomp]

I am going to try and get French Grimes to post something on combustion here. In the mean time lets move on to heads: Who wants to start? Does Velocity or Volume win races?
Velocity, up to a certain point, is a good thing. To what point depends on the port and intake shape(s). I think volume is going to be dictated by the velocity, and the MCSA of the port, if we're talking about just the head.
I'm going to email Larry.

[rmbuilder]

I'm curious about that single power stroke that is doing the real work. How much of that stroke is actually utilized?
I read somewhere years ago in my quest for info that once the crank reaches 90 degrees ATC of the power stroke, all the torque has been delivered by the crank so you might as well open the exhaust valve.
Thoughts?
Info,
Eighty percent of effective power is delivered to the arm in the first twenty percent of the power stroke. By 90* ATDC there is very little useful energy being transferred. Near this point (depending on the cylinder pressure map) the remaining cylinder pressure is better utilized in blowing down the cylinder rather than trying to extract additional power. By opening the exhaust valve (BBDC) the remaining cylinder pressure is > the pressure in the exhaust tract as the pulse exits. As the pressure excursion purges the cylinder, evacuating the residual gasses, the cylinder pressure begins to equalize. When the piston reaches BDC, the chamber is near equalized, and very little work must be done to expel the remaining gasses (on the exhaust stroke) greatly reducing pumping losses. The earlier EVO allows for the exhaust valve to begin opening sooner generating greater lift (sooner) during the exhaust stroke. Below is the timing card for the Crane 168771 cam
302*/308* @ .004”
240*/248* @ .050”
.621”/.632”
The EVO is 90* BBDC / ( 90* ATDC)
http://www.cranecams.com/index.php?s...71&lvl=2&prt=5
The more common Crane 168741 has an EVO of 86* BBDC or, 94* ATDC.
Bob

[Infomaniac]

Info,
Eighty percent of effective power is delivered to the arm in the first twenty percent of the power stroke. By 90* ATDC there is very little useful energy being transferred. Near this point (depending on the cylinder pressure map) the remaining cylinder pressure is better utilized in blowing down the cylinder rather than trying to extract additional power. By opening the exhaust valve (BBDC) the remaining cylinder pressure is > the pressure in the exhaust tract as the pulse exits. As the pressure excursion purges the cylinder, evacuating the residual gasses, the cylinder pressure begins to equalize. When the piston reaches BDC, the chamber is near equalized, and very little work must be done to expel the remaining gasses (on the exhaust stroke) greatly reducing pumping losses. The earlier EVO allows for the exhaust valve to begin opening sooner generating greater lift (sooner) during the exhaust stroke. Below is the timing card for the Crane 168771 cam
302*/308* @ .004”
240*/248* @ .050”
.621”/.632”
The EVO is 90* BBDC / ( 90* ATDC)
http://www.cranecams.com/index.php?s...71&lvl=2&prt=5
The more common Crane 168741 has an EVO of 86* BBDC or, 94* ATDC.
Bob
Thanks Bob,
That was my point with the question. Only 1 stroke really doing any work and of that stroke only a fraction of it. For sure only half of it if the exhaust valve opens around 90 degrees.
If you look at a degree wheel. Typical exhaust opening range is 100 degrees BBDC to 60 degrees BBDC. 80 degres ATDC to 120 degrees ATDC
How ineffecient of a design huh?
So what are the best ways to optimize that short event period?
And .... If for some reason cylinder pressure could be extended beyond the 90 degrees, how much useful work would it be doing?

[steelcomp]

So what are opening and closing events of an ex lobe based on? How does this affect an engine's acceleration?

[steelcomp]

If you look at a degree wheel. Typical exhaust opening range is 100 degrees BBDC to 60 degrees BBDC. 80 degres ATDC to 120 degrees ATDCat what lift is this? .006? .020? .050?
How ineffecient of a design huh? When you look at it that way, yes it is.
So what are the best ways to optimize that short event period? Create the most effecient combustion process.

[steelcomp]

The earlier EVO allows for the exhaust valve to begin opening sooner generating greater lift (sooner) during the exhaust stroke.
This is true with most "average" ex ports. This is why I'm an advocate of having the best exhaust port possible. It would be great if the ex port could flow the same as the intake port. You wouldn't have to open the ex valve so early or lift it as high. You could take advantage of the remaining pressur in the cyl, and still evacuate effeciently. The remaining pressure in the cyl past 90* is still pushing down on the piston, and anything is better than nothing.
For example:
The ex lobe in my 467, with the Canfields that I ported is opening (@.050) 69 BBDC, and closing 17 ATDC. That's a long way from the numbers you're talking about, but the ex port on those heasds flowed 298 @ .700, and 312 @ .800. That's over 85% of the intake at the same lift. The results speak for themselves. That engine also made BMRP #'s in the high 190's and broke 200 @ peak torque. That's with only 12.4:1. Good combustion.
RMbuilder...what you quoted is true to a certain extent, on certain motors, but is also a very wide and sweeping generalization.

[Infomaniac]

This is true with most "average" ex ports. This is why I'm an advocate of having the best exhaust port possible. It would be great if the ex port could flow the same as the intake port. You wouldn't have to open the ex valve so early or lift it as high. You could take advantage of the remaining pressur in the cyl, and still evacuate effeciently. The remaining pressure in the cyl past 90* is still pushing down on the piston, and anything is better than nothing.
For example:
The ex lobe in my 467, with the Canfields that I ported is opening (@.050) 69 BBDC, and closing 17 ATDC. That's a long way from the numbers you're talking about, but the ex port on those heasds flowed 298 @ .700, and 312 @ .800. That's over 85% of the intake at the same lift. The results speak for themselves. That engine also made BMRP #'s in the high 190's and broke 200 @ peak torque. That's with only 12.4:1. Good combustion.
RMbuilder...what you quoted is true to a certain extent, on certain motors, but is also a very wide and sweeping generalization.
@.050 is not where the valve opens. It has been open for .050 of lift at that point. And the cam is turning 1/2 of crank speed.
What does exhaust valve opening have to do with acceleration? Just one of the events involved with the amount of push on the piston.
Isnt that one of the directons this thread has gone? Every post has not addressed the initial question.

[Infomaniac]

Create the most effecient combustion process.
Effecient as far as?
Mechanical?
Thermodynamic?
The most power with the least amount of fuel is going to make the engine accelerate more/faster?
How about the most powerful combustion process regardless of how much fuel is used.

[steelcomp]

Effecient as far as?
Mechanical?
Thermodynamic?
The most power with the least amount of fuel is going to make the engine accelerate more/faster?
How about the most powerful combustion process regardless of how much fuel is used.
Mechanical. Thermodynamic is less an issue, although there's also debate about that. (Alum vs. cast iron heads, coatings, etc.)
Effecient dosen't necessarily mean with the least amount of fuel, to me, it means creating the best burn possible with whatever fuel you're using. The engine's effeciency (BMEP) is going to determine how much power it makes, not necessarily it's fuel consumption, although they are related. This is why I feel that the chamber, and relationship between the chamber and piston is as critical, if not more critical than any other area of focus. Too bad most guys just buy pistons off the shelf and figure that's the best they can do, without really understanding what's going on in the chamber, and in the cyl. as it fills, before compression. It's the burn that makes the power. Nothing else.
The most power with the least amount of fuel is going to make the engine accelerate more/faster? I think you'll find that an engine that will accelerate it's best can very well use less fuel than one that dosent accelerate as well, and it's certainly going to need fewer pit stops.

[Moneypitt]

[quote]I think you'll find that an engine that will accelerate it's best can very well use less fuel than one that dosent accelerate as well, and it's certainly going to need fewer pit stops.
Sounds like a plan for the next Parker enduro, have you got plans to run???MP