First, variations in peak cylinder pressure have a direct bearing on net crankshaft torque. Causes can range from variations (cycle to cycle) in air/fuel charge mixture homogeneity, transient changes in localized temperatures within the combustion space, inefficiencies in the removal of combustion residue (non-combustible exhaust gases), engine speed and load. Absent any electronic controls of the type that monitor detonation and make corresponding changes to ignition spark timing (we're dealing with race engines here, not emissions controlled), variations in cycle-to-cycle peak pressure tend to reduce net power.
So, are there any practical issues that can be addressed to minimize this condition? Well, a few might be worth considering. Research has concluded that cycle-to-cycle peak pressure variations are materially influenced by decreases in combustion rate. Fundamentally, we know that "rich" air/fuel mixtures combust more slowly than "lean" mixtures. So as an engine is made to run on the lean side of rich, so to speak, cyclic pressure variations tend to increase. However, if we're concurrently able to improve the atomization of fuel (not only particle size and uniformity but mixture homogeneity) it's possible to reach a level of compromise where combustion of a leaner mixture is made faster and we stand to eliminate some of the cyclic variations.
Following along the same line of thinking, and we've touched on this in prior columns, improvements to mixture homogeneity (atomization and mixing efficiency) not only encourage a faster burn but provide an opportunity for less initial spark timing . . . the results from which, you may recall, are higher IMEP and corresponding net torque.
Once again, putting on our practicality hat, there's another element to consider that can affect both combustion efficiency and cycle-to-cycle pressure conditions: exhaust residue. This inert material, although often containing portions of unburned fuel, does a couple of unwanted things in a racing engine. One is the dilution of fresh air/fuel charges whereby space otherwise available for combustible materials is occupied by unburnable exhaust gas. Technically speaking, this not only reduces the potential for optimizing peak cylinder pressure, but it also reduces flame temperature (useable heat), slows down flame speed, and presents a case for increasing spark timing, none of which match the combustion requirements of a race engine. EGR may be fine for emissions reduction, but that's not currently the goal for your race piece. However, don't discard the idea for some issues we'll discuss later this year.
What can be done about minimizing exhaust contamination of fresh mixtures? An efficiently designed exhaust system in the range of most operated rpm is a place to start. For example, if your engine is most often run in a 4,500-7,500 rpm span, sizing a system to be optimally efficient in this range will help improve the blow-down portion of the exhaust cycle.
Depending upon the point of exhaust valve opening, peak blow-down pressure will occur at different engine speeds and crankshaft angles, but still rely on the sizing of an exhaust system that matches the desired rpm range, particularly the peak torque rpm point. Making certain that a given system is most efficient in this span of engine speed will help minimize combustion residue present at the arrival of each fresh air/fuel mixture charge. Of the conditions this benefits, not slowing down the combustion rate from air/fuel charge contamination trails back to the need for minimizing cycle-to-cycle peak pressure variations. And we've come full circle.
What we'd like to leave you with this month can be boiled down to a couple of issues. The building of a successful race engine is a function of understanding as many of the factors that influence its performance as possible. Two extremes of information populate this requirement: one is purely academic and woven with theoretical and scientific perspectives, and the other is based on pure experience in the building and evaluation of race engine packages. Smokey often told me, you need some of both.
One of the purposes of this column is to attempt a blending of those extremes. To draw benefit from the information Enginology is designed to contain, if I'm able to do this job effectively, requires more than just casual reading. Unless your responses to CT's Editor, Rob Fisher, are to the contrary, we'll begin placing more emphasis on these two informational components: theory and practical applications. By design, this approach will hopefully help you understand the "why" underlying the "what" as you continue the search for more understanding about engine basics and how they can be transposed into winning race engines.