HCCI engines offer the promise of significant fuel consumption benefits (~15 – 20%) relative to spark-ignition (SI) engines; low levels of NO_x emissions compared to SI or diesel (compression ignition—CI) engines; and extremely low particulate emissions compared to diesels.

In an HCCI engine, fuel and air are mixed together and injected into the cylinder. The piston compresses the mixture until spontaneous combustion occurs. The engine thus combines fuel-and-air premixing (as in an SI engine) with spontaneous ignition (as in a diesel engine). The result is HCCI’s distinctive feature: combustion occurs simultaneously at many locations throughout the combustion chamber.

With combustion occuring throughout the combustion chamber in HCCI, there is no need for a quickly spreading flame to combust the fuel charge before a new charge enters the chamber. As a result, combustion temperatures can be lower, so emissions of oxides of nitrogen are negligible (although hydrocarbon emissions are higher). The fuel is spread in low concentrations throughout the cylinder, so the soot emissions from fuel-rich regions in diesels are not present.

However, managing combustion in the HCCI process is difficult, especially given different operating conditions and transients. Specifically, precisely controlling combustion phasing is difficult in the HCCI combustion regime due to the lack of an inherent control strategy, such as spark- or injection-timing, used in traditional engines.

According to Professor William H. Green, Jr. of MIT’s Department of Chemical Engineering, ignition timing in an HCCI engine depends on two factors: the temperature of the mixture and the detailed chemistry of the fuel. Both are hard to predict and manage outside of controlled conditions in the laboratory.

The range of conditions suitable for HCCI operation is far smaller than the range for SI mode. Variations in temperature had a noticeable but not overwhelming effect on when the HCCI mode worked. Fuel composition had a greater impact, but it was not as much of a showstopper as the researchers expected.

In work described in one paper presented at the meeting, “Effects of Variations in Market Gasoline Properties on HCCI Load Limits”, the research team, which included Ford Powertrain Research and BP, measured the impact of market-fuel variations on the HCCI operating range in a production Mazda 2.3L, in-line, 4-cylinder, 16-valve engine, modified for single-cylinder operation.

The intake air and exhaust from the firing cylinder were kept separate from the motoring cylinder flow to ensure accurate fuel/air ratio measurements. Other modifications to the engine included: increasing the compression ratio from 9.7 to 11.1; adding continuously variable cam phasing to the exhaust valve train (continuously variable intake cam phasing was a standard feature on the production engine); and using cams with reduced durations and lifts.

To induce HCCI combustion, the MIT engine uses negative valve overlap (NVO)—the exhaust valve closes early during the exhaust stroke to trap hot exhaust gas residuals. The thermal energy of the exhaust gases induces auto-ignition on the subsequent cycle. The use of cam phasing maximizes the HCCI operating range in terms of the high load limit (HLL) and low load limit (LLL) at each engine speed.

When approaching low load limit, the goal was to minimize fuel consumption. The amount of fuel inducted was directly related the amount of residual mass in the cylinder; higher residual fractions resulted in lower fresh charge fractions. To increase the residual fraction, the exhaust cam phasing was advanced (i.e. exhaust valve closing, EVC, occurred earlier during the exhaust stroke).

At the HLL, the goal was to maximize torque output from the engine by increasing the amount of fresh fuel and air inducted during the intake process. To support more fresh charge, the exhaust gas residual fraction must decrease. This was accomplished by retarding the exhaust cam phasing, i.e. EVC timing was pushed later in the cycle. Combustion phasing to control the pressure rise in the cylinder was retarded by adjusting the intake cam to control the amount of fresh charge entering the cylinder.

The researchers tested 12 different fuels blended from commercial refinery streams, which were designed to span the market-typical variation in the select fuel properties: volatility (measured by the Reid vapor pressure: RVP); Research Octane Number (RON); aromatic content; olefin content; and ethanol content.

The researchers concluded that:

• The effects of market-typical changes in fuel composition on the operational limits were modest; to a first order approximation, all fuels achieved nearly equal operating ranges. This conclusion is only supported for NVO-induced HCCI. “Employing other controlling methods of HCCI combustion may result in larger and more discernible fuel effects on the HLL.”

• The effect of adding ethanol to the test fuels was insignificant at the LLL and HLL.

• At the high-load limit, some small fuel effects on the operating range were observed; however, the observed trends were not consistent across all the speeds studied.

• Within experimental measurement error, there was no change in the LLL among the blends of test fuels.

This research was supported by Ford Motor Company and the Ford-MIT Alliance, with additional support from BP.

Resources:

• Andreae, M.M., Angelos, J.P., Cheng, W.K., Green, W.H., Kenney, T., and Xu, Y.; “Effects of Variations in Market Gasoline Properties on HCCI Load Limits”; JSAE 20077050 / SAE 2007-01-1859

• Managing transient behaviors of a dual mode spark ignition-controlled auto ignition engine with a variable valve timing system

• Andreae, M.M., Cheng, W.K., Kenney, T., and Yang, J., “Effect of Air Temperature and Humidity on Gasoline HCCI Operation,” SAE 2007-01-0221