Colyn,

 

Note: engines with the same displacement (i.e. 550 ci) do not necessarily all have the same mass airflow and hence not the same fuel flow at peak EGT (and hence not the same HP, Compression Ratio not withstanding).

 

1) For example, a turbocharged (or supercharged) engine can achieve a higher mass airflow using the same volumetric displacement because the manifold pressure (air density) is higher.  Thus, the fuel flow will also be higher at peak egt (same mass based air/fuel ratio).

 

2) Likewise induction and exhaust tuning/porting can change the volumetric efficiency.  For example, in a 550 ci engine, typically some % less than 550 ci of new air is actually ingested during each cycle due to induction and exhaust inefficiency.  Explanation of volumetric efficiency below…

 

To begin thinking about volumetric efficiency, think about how an engine is controlled.  Given: a 550 ci engine with a constant rpm governor/prop.  To reduce (i.e. control) power, we use the throttle to partially stave the engine of air (and fuel).  During each complete cycle (2 rotations), the pistons always displace 550 ci of volume.  However, less that 550 ci of “ambient” air is consumed.  Rather, the ambient air is expanded (lower pressure) as it squeezes past the throttle restriction (butterfly).  Thus, at half throttle, perhaps only 275 ci of ambient air is actually consumed.  In this scenario, the pistons see 550 ci volume of air flow (but at a reduced pressure and density).  Whereas the air filter only see’s 275 ci volume of ambient pressure air flow.

 

Ideally at WOT (wide open throttle), the pistons and air filter would both see 550ci of flow (displacement).  However, due to inefficiencies in the intake and exhaust flows the air flowing through an engine is typically still “throttled” somewhat and the air filter see’s something less than 550ci of flow.

As the piston goes down, the pressure in the cylinder must be less than that in the intake manifold to get the air moving in.  Ideally, at the bottom of the intake stroke, the intake value would be left open long enough for the air in manifold and cylinder to equalize (maximum fresh air in).  However, the crank and piston have other plans (they don’t stop moving).  At higher RPM, the time for intake equalization is reduced.  Furthermore, it takes some time (degrees of rotation) for the intake value to move from fully open to fully closed.  So, it must start closing early.  Thus, at the bottom of the intake stroke the cylinder is often left a bit deficient.  The “free-er” the intake flow, the less deficient the cylinder at the end.  This phenomenon is minimized by leaving the intake value open somewhat past BDC (controlled by the camshaft).  However, too much delay, and the piston is moving up and trying to push the air back out and compression is impacted.  In a perfectly “tuned” system, the intake flow is accelerated while the piston is moving down, such that as the piston nears the bottom, the intake air (in the manifold / runners) has momentum which helps carry/force it into the cylinder in the final moments of that cycle.

 

During the exhaust stroke (while the piston is moving up), the pressure in the cylinder has to be greater than in the exhaust manifold (headers) in order to get the exhaust air moving out.  Near the top of the exhaust stroke the exhaust value starts to close - note it takes time (rotation) for the value to close.  During this time, some of the (pressurized) exhaust air gets caught in the cylinder as the exhaust value is shutting.  The “free-er” the exhaust flow, the less exhaust that gets “trapped” in the cylinder.  To help minimize this phenomenon, the closing of the exhaust value is often designed to occur at a point somewhat just beyond piston TDC (top dead center).  During this time the intake value is also opening (since that also takes time/rotation).  The period where both exhaust and intake are open (partially open) is known as overlap.  If the overlap is too large, it affects the efficiency of the following intake stroke.  Likewise, delays in opening the intake value also effect efficiency of the intake stroke.  So like everything there are tradeoffs.  In a perfectly “tuned” system, the exiting exhaust flow is accelerated while the piston is moving up, such that as the piston nears the top, the exhaust has momentum which also helps carry it out of the cylinder in the final moments of that cycle (aka. tuned exhaust).

 

 

 

Peak EGT always occurs at a F:A ratio of about .067... period. The FF at that peak is based on the mass airflow, not the CR of the engine.