Machine Design

Don't destroy that race-car engine. Use this software instead.

In the past, simulating race-car performance was usually the bailiwick of dedicated analysts running high-end, expensive equipment.

Chris van Rutten

Edited by Leslie Gordon

A chart in LapSim shows the resulting airflow of the intake valves, dependent on valve lift. Airflow is calculated from input parameters such as valve angles, lengths, and diameters selected by the user.

The chart on the right shows how long the cam dwells at several valve-lift heights. The graph on the left plots acceleration per degree and absolute acceleration at the rev limiter.

The graph on the left shows an ignition reduction curve for full load at 5,500 rpm for a particular engine. The graph on the right shows the burn sequence. The blue line represents temperature of the unburned mixture; the yellow line is pressure in the cylinder; and the red line is average temperature of the burned and unburned mixture. The gray area shows the amount of mixture burned.

A 3D graph shows engine torque as a function of engine rpm and throttle angle.

No longer. The new LapSim Engine module, which works with LapSim V.2007 vehicle-simulation software, takes only 34 parameters to simulate any four-stroke, spark-ignition race engine from a street-legal sports car engine like a Porsche 911 or BMW M3 to a 20,000-rpm Formula 1 engine, with or without air restrictors, on a PC. The software claims at least 95% accuracy with less than 5% of the effort of complex engine simulation packages.

To begin an engine simulation, users must first determine intake-valve resistance. This is critical because the resistance limits airflow into the cylinder, and thereby the amount of air it captures after the valves close. LapSim follows a rather unconventional route, which took a lot of effort and knowledge to develop. The software estimates the resistance of a valve dependent on its lift, size, and position. Users need only select a few parameters such as valve angles, lengths, and diameters to get a good flow curve, without depending on flow-bench tests. This saves a lot of effort, especially when no engine is yet available, like in the design or concept phase.

The next step is to relate valve position to crankshaft angle. Users do this with an adjustable camshaft diagram, in which they select duration, lobe centre position, and opening and closing acceleration. Users also specify valve clearance, as well as intake and exhaust timing. An option lets users input how long the cam dwells at maximum lift. This targets engines in which maximum lift is limited by regulation. Again no real camshaft data is necessary to run a simulation.

With valve motion specified and valve resistance determined, the next step is modeling intake and exhaust dynamics. The goal of the model is to generate a cylinder-pressure diagram over the full 720° of crankshaft rotation.

Users select stroke, cylinder bore, and connecting-rod length. These parameters determine the exact position of the piston and also the cylinder volume at any crank angle. They also determine piston speed. The software represents intake and exhaust dynamics as spring and mass systems, with properties defined by lengths and diameters of the intake runners and primary exhaust branches. This is a simplification of manifold configuration, but again the gain in user friendliness far outweighs any losses in accuracy.

Plotted results let users see where valve resistance and the time available from cam duration leave the cylinder "short of breath" in the cycle. Changing intake dimensions and cam timing alters the cycle characteristics.

The next step is calculating compression and expansion strokes. Combustion begins at the end of the compression stroke. Combustion is difficult to model because it is neither an instantaneous nor a constant process. The software uses a rather elaborate burn model with two zones, a burn and a fresh-mixture area. Each zone has different temperatures associated with the degrees of freedom.

The two-zone model means the form and size of the chamber significantly influences burn progression. A smaller bore, for instance, will lead to shorter burn duration.

A bad chamber shape will lengthen burn duration and reduce efficiency. Users determine the shape of the combustion chamber by choosing bore, stroke, and compression ratio, as well as chamber height and radius.

The model also lets designers calculate the influence of advancing or retarding ignition. Plotted results let users compare, for instance, normal timing to retarded timing and see how it affects torque. Users can also explore knocking (where burn speed is too fast and pressure rise becomes too high) to see how far to advance the ignition for improved torque and efficiency gains — without destroying the engine.

Once combustion is modeled, the software generates a cylinder-pressure curve over the full 720° of crankshaft rotation. Plotted results include color-coded pressure diagrams for several rpm levels. The different colors represent the burn phase, overlap between intake and exhaust valves, exhaust-open phase, and maximum cylinder pressure.

Lastly, the software generates an engine-torque curve for the entire rpm range. Designers can use the power curve in the lap simulation model to calculate a lap time for the engine mounted in a specific car.

The software comes from Bosch Engineering GmbH, Markgroeningen, Germany, A free version of LapSim is available for download at

Chris van Rutten can be reached at [email protected]

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