There has never been a performance engine builder who isn’t constantly looking for extra horsepower. Until now, the dynamometer provided all performance data, but no longer. Combustion analysis (CAS) is now the ultimate measurement tool. While cylinder pressure measurement has been around for some time, it is only in the past few years the performance benefits have surfaced. In one baseline test, information can be revealed that can never be found in continuous dynamometer testing; information that can save many hours of trial and error.
A combustion analysis system (CAS) measures individual cylinder pressure every degree of crankshaft rotation, at any chosen rpm. Pressure is measured by one of two types of transducers, both extremely accurate. One is a special designed spark plug; the other is for permanent mounting. Pressure is normally measured over seventy five to one hundred engine cycles; one cycle being 720 degrees of engine rotation.
Previously to benefit from collected data, complex mathematical heat release calculations were required or the utilization of MatLab or similar programs. Now, software programs have been developed to interpret all data; on an individual cylinder basis, and print a report showing both graphical and analytical results minutes after the test.
How do we analyze each cylinder?
As I have said before combustion is a misnomer; combustion analysis implies the involvement of thermodynamics, which it does, but only to a small degree. Individual cylinder pressure measurement is what we are dealing with. A baseline test is the beginning of CAS engine development. The engine is mounted on the dynamometer, intake air is set for operating altitude and several “pulls” are made to establish operating temperature, torque and horsepower. Next, transducers replace sparkplugs and data is sampled at desired rpm; usually max torque and max horsepower. Sample time is 75 to 100 cycles. Data is downloaded and a report is printed in both graphical and analytical format. This requires approximately 10 minutes. I need to say now this report, format and all graphs are not standard. They are the results of a proprietary software program owned by iSystems, Inc; the sole purpose is to save time disseminating the data. Now we review cylinder pressure. We look at each cylinder in several ways; peak pressure, cyclical variation, pressure rate of rise measured in crank angle degrees, mass burn and IMEP, “indicated mean effective pressure”; the pressure that is pushing on top of the cylinder. The baseline test will show “traces”; lines if you like, of each cylinder during the test period. These can be displayed individually, collectively or in bar chart form. (See sample.) The desired goal is to have ALL traces coincide and horizontal; i.e. one flat line. How do we get there? Not to sound vague but there are many avenues; individual ignition trim, altering combustion chamber, piston dome, rod length, cam lobe profile or air fuel ratio to mention a few. There is NO one magic bullet! Depending on the degree of variation, one modification may be tried first; EX. The 50% Mass Burn graph shown previously, individual cylinder ignition trim would be a very good choice. Results would be instant. The most compelling testimony for cylinder pressure measurement would have to be the NASCAR open cup engine. That engine has 360 cubic inches, limited to 12:1 compression, 9300 rpm and breathes through one four barrel carburetor with specified venturi, a few teams have hit 900 reliable horsepower!
Combustion analysis is the quickest, least expensive, and the most accurate method of engine development! It allows the individual to analyze empirical data taken from a running engine that will be the basis of a blueprint for maximum efficiency and power. It is beneficial not only to gasoline engines but also to diesel and alternative fuel.
Editor’s Note: This is the first of a multi-part series. The next will be a data review of an actual CAS test on an engine supplied to iSystems, Inc. For more information, call (406) 587-9369, e-mail iSystemsinc@gmail.com or go online: www.isystemsperformance.com.
Levon Pentecost is a graduate of Auburn University in Industrial Engineering. He has been involved in fields as diverse as manufacturing design systems to medical devices and engine design and development. He has been involved in motorsports since the 60s and has raced in IMSA, 24 Hours of Daytona and 12 Hours of Sebring. He has built and developed motors for high-profile clients such as the late Bob Snodgrass, president of Brumos Motorcars, and Jim Bailie, manager of Brumos Racing, as well as many others.
Cecil Stevens has been developing V8 push rod valve control systems for maximum performance for over 19 years. He brings expertise in in-cylinder combustion pressure, testing and analysis, and test equipment and software; and 16 years experience in NASCAR Cup racing with two teams. His achievements include: development of NASCAR Dodge and Toyota Motorsports engine valve train with combustion analysis, and 151 NASCAR Cup races without a single valve train failure.
For a PDF of this article (complete with photos), go to: