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Small internal combustion engines outperform batteries and fuel cells in regards to weight for a range of applications, including consumer products, marine vehicles, small manned ground vehicles, unmanned vehicles, and generators. The power ranges for these applications are typically between 1 kW and 10 kW. There are numerous technical challenges associated with engines producing power in this range resulting in low power density and high specific fuel consumption. As such, there is a large range of engine design solutions that are commercially available in this power range to overcome these technical challenges. A market survey was conducted of commercially available engines with power outputs less than 10 kW. The subsequent analysis highlights the trade-offs between power output, engine weight, and specific fuel consumption.

In 2020, Cheeseman (SAE Paper 2020-01-1314) introduced Entry Ignition (EI) as a potential engine combustion process to rival traditional Spark Ignition (SI) and Compression Ignition (CI). The EI process premixes fuel with compressed air, which then enters a hot cylinder at top dead center, autoigniting upon entry. The original proposed concept for an engine separates the compression and expansion processes allowing for it to be modeled as a 2-stroke Brayton cycle. Theoretically, an EI engine allows for higher compression ratios than SI engines with less emissions than CI engines. However, the original EI engine analysis made several assumptions that merit further investigation. First, the original analysis did not look at the temperatures and pressures in the air/fuel mixing chamber to ensure that it does not autoignite prior to entering the cylinder. Second, the analysis did not account for the large amount of heat transfer associated with keeping half the end-gas in the cylinder.

It is widely known that the rapid autoignition of end-gas will cause an engine cylinder to resonate, creating a knocking sound. These effects were quantified for a simple engine geometry in 1934 in a study where critical resonance frequencies were identified. That analysis, performed by Charles Draper, still forms the basis of most knock studies. However, the resonance frequencies are highly dependent on the engine geometry and the conditions inside the cylinder at autoignition. Since, engines and fuels operate at substantially different conditions than they did in 1934, it is expected that there should be a shift in knock frequencies. Experimental tests were run to collect knock data in an engine, representative of modern geometries, over a range of operating conditions for a number of different fuels. The operating conditions-intake air temperature, intake air pressure, and engine speed-were varied to identify shifts in the critical frequencies.

Experiments were carried out to collect in-cylinder pressure data and microphone signals from a single-cylinder test engine using spark timingsbefore, at, and after knock onset for toluene reference fuels. The objective was to gain insight into the phenomena that determine knock onset, detected by an external microphone. In particular, the study examines how the end-gas autoignition process changes as the engine's spark timing is advanced through the borderline knock limit into the engine's knocking regime. Fast Fourier transforms (FFT) and bandpass filtering techniques were used to process the recorded cylinder pressure data to determine knock intensities for each cycle. Two characteristic pressure oscillation frequencies were detected: a peak just above 6 kHz and a range of peaks in the 15-22 kHz range. The microphone data shows that the audible knock signal has the same 6 kHz peak.