Mechanical Engineering Department
BS in Mechanical Engineering
This project, engine development, was sponsored by Cal Poly FormulaSAE. The FormulaSAE team proposed this project in order to have a more reliable and more powerful engine for their car, to improve their overall performance at their competition. The baseline output of the engine is 37 hp and 24 lb-ft of torque. The goal of this project was to increase the output to 45 hp, to meet requirements determined by a trade study. We planned to increase the engine’s output using several strategies.
We evaluated all potential design decisions using a Ricardo WAVE model. Ricardo WAVE is a software tool that can simulate the operation of engines and their components. Using accurate measurements of important engine parameters, WAVE simulated the results of engine dyno testing, allowing us to narrow down our design choices quickly and inexpensively. The model predicted a baseline of about 40 hp and 30 lb-ft of torque. When comparing the WAVE model and the baseline dynamometer test of the engine for model verification, the power curves were similar but with the simulation curve being shifted toward higher engine speeds. The model predicted higher power because the model shows power at the flywheel while the dynamometer measures power at the brake, after the gearbox and sprockets. The model was also used to predict the effects of increasing the engine’s compression ratio as well as the effect of different camshafts and valve timing.
By carefully determining gear ratios and rear sprocket size we allowed the car to have as much acceleration as possible in each driving event. The amount of force that the car transmits to the ground depends directly on the gear ratios in the transmission and the ratio of the engine sprocket to the axle sprocket. Thus, gearing is an important aspect of overall car performance and is set up to match the powerband of the engine.
In order to increase power the camshaft profile and timing were both altered in order to provide a powerband more suitable for the FormulaSAE competition. The current powerband is designed for the high revving endurance motorcycle where top speed is critical and the intake is unrestricted, while FormulaSAE places a premium on power at lower speeds to help the car accelerate out of corners. Another major reason for bringing the powerband down lower is that at lower engine speeds, the effects of the restrictor are minimized when compared to higher engine speeds.
The engine intake directly affects the ability of the engine to intake air. More air in the cylinder means more fuel burned and more power, so devising a way to maximize the amount of air in the cylinder is an excellent way to increase engine power. As the engine is essentially sucking air in from the atmosphere, nearly anything that reduced head loss in the intake tract was beneficial. Wave dynamics were equally, if not more, important in intake design. By careful analysis of pressure wave dynamics, the intake tract was optimized for increased power at a chosen engine speed.
As with the intake system, the exhaust system focused on reducing pumping losses and harnessing wave dynamics to increase engine performance at a specific engine speed.
All of these efforts together give the FormulaSAE team the performance it needs to perform well at the FormulaSAE competition in Detroit. As a result of our efforts, the engine makes 42 horsepower at peak, which is a 13.5% increase. The engine’s peak torque is now 27.5 lb-ft, which is a 14.5% increase. The engine also has a 10% increase in power, as well as a 12.5% increase in torque across the powerband, as we shifted the powerband to a lower engine speed.