Cylinder psi vs load
 

Cylinder psi vs load.

Marine engines run in heavily loaded conditions - what effect does this have on cylinder pressure ?

Thoughts are welcome. :D

Re: Cylinder psi vs load
 

A guess; the cylinder pressure will mainly depend on how much air and fuel that gets in to the cylinders during the intake cycle and of course the compression ratio. The amount of air that enters the cylinders is dependant on the manifold air pressure which in turn is dependant on the load. When you cruise in a car ther is not much load but when you step on it you basically have atmospheric pressure in the manifold creating the same load and cylinder pressure as you have when hammering in a boat given the same compression ratio.

Re: Cylinder psi vs load
 

cylinder pressure is the result of several converging timelines of factors that are part of the ignition/combustion process.

Cylinder filling has its own graph. It is affected by intake cam lobe profile, intake cam opening and closing events, port length, diameter, and flow characteristics, plenum chamber volume, throttle plate angle, air density, valve size, combustion chamber shape, piston dome profile, and exhaust scavenging action. Compounding this is that all of the above are all constantly changing at each different rpm and whether the motor is in a steady state status or is accelerating.

Ignition and Burn cycle has its own graph as well. It takes the cylinder filling graph, the exhaust scavenging graph, the changing percentage of oxygen in the chamber, the spark initiation point and its duration, the placement of the spark in relation to the incoming charge, the momentum and directional inertia of the incoming charge, the swirl or turbulence of the charge, the flamefront travel of the initiated burn, the surface area of the chamber and the piston, the squish area of the chamber, and the burn speed of the fuel.

The exhaust scavenging graph is also a factor, as it affects the available volume for incoming charge, affects the oxygen content of the chamber's contents, and offers a buffering effect on the degree of explosive nature of the initiated burn cycle.

Engine load and piston speed curves also afect the dynamically changing size, shape, and surface area of the combustion chamber. A single cylinder motor with very little crankshaft inertia and no load will respond instantly to the ignition cycle, thus opening the chamber very quickly. This would keep the ultimate cylinder pressure lower than an engine that was loaded, had heavy crank weighting, and a host of other cylinders smoothing the cycle. In those cases, the engine would be less responsive to clearing the chamber, and the burn would be more apt to spike pressures.

Take all of these curves and plot them on top of each other and you can start getting a mathematical representation of what your cylinder pressure will be at a particular "state".

The literature in the cam book is very very superficial and only is to be used as a guide. Going by their wording, they appear to say bigger is better. This is never the case.

The simple version of the situation is that a boat cannot rev out between shifts like a car. The faster the motor spins, the steeper the hill is you are climbing. A pure loading curve.

So why worry about cylinder pressure? Go too high, you detonate. Ping.
The goal is to maximize volumetric efficiency at all engine speeds, optimize fuel mix and ignition timing to give an efficient fuel burn and close to optimum cylinder pressures.

But it's all give and take. Can only optimize for a certain range. The rest is a compromise.

mc

Re: Cylinder psi vs load
 

M, That's an interesting read. Where did you learn this?
jb

Re: Cylinder psi vs load
 

Apart from the static compression ratio, the intake valve closing will have a significant effect on the cylinder pressure, since preassure will bleed of during the compression stroke.

Re: Cylinder psi vs load
 

MC - all very good / pertinent info !

The above quoted statement is the one I was concentrating on.

I believe Crane was talking in a sense if the engine is already built a certain way - as if some sort of higher compresion crate motor. They concentrate on camshafts of course.

I wanted to bring this topic up just to make some think about something that some don't think about or not know about. It is this resistance of the reciprocating assembly to turn faster that creates some very crucial aspects of cyl psi from combustion.

Most think cyl psi is in regards to compression only so I jujst wanted to remind / show people that combustion pressures are something that we should be really concerned about.

Remember, the force on the piston from combustion psi is the same on the cyl walls + cyl head. It's the reciprocating assembly that has give (movement) and thus transfers most of the resultant energy out of the motor into driveshaft/outdrive/pro/water. If the engine is loaded enough where it is very resistant to movement then where is all this energy going ? :eek:

We are not in an environment like most cars where we are able to have an engine lightly loaded where it can pass thru the rpm range quickly.

mcollinstn - thank you again. That was great information and shows the many areas that effect running cyl compression psi and cyl combustion psi. All must be looked at and accounted for when building an engine to produce good power + dependability when stuck in a boat.