The Effects of Intake Plenum Volume on the Performance of a Small Naturally Aspirated Restricted Engine

Intake tuning is a widely recognized method for optimizing the performance of a naturally aspirated engine for motorsports applications. Wave resonance and Helmholtz theories are useful for predicting the impact of intake runner length on engine performance. However, there is very little information in the literature regarding the effects of intake plenum volume. The goal of this study was to determine the effects of intake plenum volume on steady state and transient engine performance for a restricted naturally aspirated engine for Formula Society of Automotive Engineers (FSAE) vehicle use. Testing was conducted on a four cylinder 600 cc motorcycle engine fitted with a 20 mm restrictor in compliance with FSAE competition rules. Plenum sizes were varied from 2 to 10 times engine displacement (1.2–6.0 l) and engine speeds were varied from 3000 rpm to 12,500 rpm. Performance metrics including volumetric efficiency, torque, and power were recorded at steady state conditions. Experimental results showed that engine performance increased modestly as plenum volume was increased from 2 to 8 times engine displacement (4.8 l). Increasing plenum volume beyond 4.8 l resulted in significant improvement in performance parameters. Overall, peak power was shown to increase from 54 kW to 63 kW over the range of plenums tested. Additionally, transient engine performance was evaluated using extremely fast (60 ms) throttle opening times for the full range of plenum sizes tested. In-cylinder pressure was used to calculate cycle-resolved gross indicated mean effective pressure (IMEPg) development during these transients. Interestingly, the cases with the largest plenum sizes only took 1 to 2 extra cycles (30–60 ms) to achieve maximum IMEPg levels when compared with the smaller volumes. In fact, the differences were so minor that it would be doubtful that a driver would notice the lag. Additional metrics included time for the plenums to fill and an analysis of manifold absolute pressure and peak in-cylinder pressure development during and after the throttle transient. Plenums below 4.8 l completely filled even before the transient was completed.