- Additional Engine Comparison Tools -
A Summary of Performance Comparison Yardsticks
In previous sections, we have covered the fundamental yardsticks for doing insightful comparisons of engine performance. The Thermal Efficiency section discusses the basic relationships between fuel consumed and power produced, and provides a method for evaluating engine performance claims. The Volumetric Efficiency section discusses methods for realistic evaluations of power capacity and another useful tool for evaluating performance claims. The BMEP section explains one of the most fundamental tools for evaluating performance and comparing performance among dissimilar engines.
This section introduces another tool for comparing the performance of piston engines.
Engine Performance Coefficient
Stepping back to engine fundamentals, we know that the potential power any engine can produce is directly dependent on two factors:
- The mass of air it can ingest per second, and
- The BSFC it can coax from the fuel.
The BSFC parameter encompasses elements including the heat content of the fuel, best-power air-fuel ratio, thermal efficiency, mechanical efficiency, mixture homogeneity, mixture motion, chamber design, combustion quality, and others.
The mass airflow parameter encompasses elements including bottom-end design (RPM capability), runner, port, valve and chamber design, cam profile and valvetrain design, and others.
Mass airflow is dependent on:
- Air density and
- Volumetric efficiency (VE).
As described in the section on Volumetric Efficiency, at 100% VE, the volume of air a four-stroke engine can ingest is proportional to: RPM x Displacement ÷ 2. The following relationship expresses that potential airflow as a dimensionless number (Potential Airflow Number, PAN) as:
Potential Airflow Nunber = (rpm / 1000) x (displacement / 2)
It is a revealing insight into engine performance to examine the relationship between power produced and potential airflow. We will now define an empiricism which clearly expresses that relationship. Let's name it Engine Performance Coefficient (EPC) because it provides another basis (in addition to BMEP, BSFC, MPS and BHP/Cubic-Inch) for comparing one engine to another.
EPC = Peak Power / Potential Airflow Number
Combining terms and rearranging the equation produces:
EPC = (Peak Power x 2000) / (rpm x displacement)
The EPC factor encompasses all the engine design variables and provides a basis for comparing two totally different engines on the basis of how well they convert fuel into power. For example, at peak power, the 2006 version of the 2.4-liter Formula 1 V8 engine (pre-19,000 RPM rev limit) produces roughly 755 BHP at 19,250 RPM. The EPC for that operating point is:
EPC = 755 x 2000 / (19,250 x 146.46) = 0.536
Now let's look at the EPC for a NASCAR Sprint Cup engine from the same time period. That engine (pre-gear-rule) produced about 825 BHP at around 9000 RPM. The EPC for that operating point is:
EPC = 825 x 2000 / (9000 x 357.65) = 0.513
It is quite surprising to discover that the EPC figure for the Cup engine is only 4.3% less than the F1 engine, especially in view of the fact that the F1 engine is a purpose-designed DOHC, 4-valve race engine with few restrictions, while the Cup engine is a severely restricted, nominally production-based iron block V8, with two-valves per cylinder, pushrod / rocker arm valvetrain, 0.875 diameter flat tappets, single central carburetor, and more.
Considering the restrictions, the small 4.3% difference in EPC between Formula One and Cup gives a real insight into just how clever the Cup engine people really are.