photographer: Charles H. Jenckes
In this segment of the Series, you're provided some considerations directed toward component selections that form the "foundation" of a high performance or racing engine. To round out an understanding of how such choices may affect the combustion process and its ability to optimize power with reliability, some additional aspects need to be discussed.
Of the complexities woven into an internal engine's combustion process, one is the fact this occurs during a rapidly changing combustion volume and highly variable pressures. In addition, there will be a certain amount of surface area exposed to combustion, causing some measure of otherwise useable heat (power) to be lost. Various factors will influence how the process not only develops but is maintained during the cycle.
For example, consider changes to engine geometry that affect piston speed. These include both crankshaft stroke and connecting rod length. Aside from how these dimensions affect lateral thrust loading on a piston, the rate of piston acceleration and residence time along any given position in its stroke will also be altered. Consequently, the rise of cylinder pressure and the duration of the pressure becomes a function of rod length and crankshaft stroke. And, from a combustion perspective, flame speed, and ultimately power output, will be affected.
While it is generally known that increasing connecting rod length correspondingly tends to increase piston residence time at BDC and TDC positions, the rate at which cylinder pressure rises (as a function of crank angle) during the compression stroke and pre-TDC burn will differ from that when using a shorter rod. Even viewed in its simplest form, this condition is one that can be modeled using engine simulation software to include other variables in determining "best case" parts combinations.
Further, with respect to changes in connecting rod length, data supports the notion that an increase in length tends to move peak power nearer to the peak torque rpm. On tracks where engine speed is characterized by a narrow range of rpm, this may be desirable. As a wider span of on-track rpm becomes useful, spreading torque and horsepower peaks over a broader range, by shortening rod length, might help. Interestingly, we know that piston residence time (at TDC beginning the power stroke) with longer rods tends to increase, thereby allowing more heat to be lost into the cylinder heads and cooling system. From a thermodynamic standpoint, this additional heat "soak" can be detrimental to power. However, longer rods present less side loading (in the cylinders), reducing losses to friction. And, from a mechanical standpoint, there are tradeoffs to consider. Above all, it's critical to work with valve timing and events to optimize the benefits of slow (long rod) or rapid (short rod) movement of the piston during the initial stages of the inlet cycle.
Overall, flame speed tends to increase with engine speed. Generally speaking, largely due to turbulence in the inlet track and combustion space, in-cylinder pressure measurements (using Engine Cycle Analysis) have shown that a major portion of the combustion process occurs at nearly constant crank angles; e.g., near, at, and just beyond TDC during combustion. In fact, peak pressure will occur just past TDC on the power stroke and becomes delayed further as rpm increases. This points to the importance of making certain pre-combustion mixture quality is optimized, in order to minimize combustion break-down (detonation, misfire, etc.) during this critical time. It's unavoidable that the highest level of combustion efficiency must take place in increasingly shorter periods of time as higher rpm is reached.