A turbo-charged, spray-guided direct-injection engine running on pure ethanol (E100) can achieve very high specific output, and shows “significant potential for aggressive engine downsizing for a dedicated or dual-fuel solution”, according to engineers at Orbital Corporation.
Orbital developed an E100-fueled, four-cylinder, turbo-charged direct injection engine using its air-assisted, low-pressure direct injection technology (earlier post) as a platform for investigating the potential for high specific output with ethanol.
The concept of enabling higher gasoline ICE operating efficiency through higher specific output and down-sizing is well reported in the literature. It is typical that turbo-charging is utilised to efficiently achieve high engine airflow across the engine speed range, and is coupled with direct injection to facilitate low end performance and increased compression ratio. Nonetheless specific output is still constrained by the combustion characteristics of gasoline, and whilst this may be partially offset by dilution strategies it remains the case that compression ratio and output are both limited.
It is anticipated that the operation of a turbo-charged DI engine at high load on pure ethanol may resolve these issues and deliver exceptional output, thereby enabling a more effective use of ethanol as a renewable fuel.—Simon Brewster, “Initial Development of a Turbo-charged Direct Injection E100 Combustion System”
The injectors were centrally mounted in close proximity to the sparkplug.
The air-assist injector decouples the fuel metering and delivery events, thereby assisting the dynamic range of the injector and making it suited to boosted applications, particularly for ethanol which requires significantly higher fuel flow rates than gasoline. The low SMD [Sauter Mean Diameter] [in the order of 10µm and less] is understood to offer advantage in low temperature starting for low volatility fuels, and is therefore proposed to offer benefit for low temperature starting of ethanol.
Although designed for boosted operation on gasoline, the engine was not designed for unusual specific output, and was therefore load limited under certain conditions. Orbital performed comparative testing of ethanol and gasoline fuels in the engine at a compression ratio of 10.4:1, and under three operating scenarios:
Under high load at output typical of a boosted gasoline engine;
At extended output levels at lower and higher engine speeds; and
For start and light-off performance at a temperature of 25°C.
Among the key findings of the study were:
At the same ignition timing and intake manifold pressure, ethanol demonstrated higher output which is attributed to higher airflow, more advantageous combustion phasing and lower heat loss.
At the same ignition timing and output, ethanol required lower airflow and boost pressure, and delivered lower exhaust temperature, higher brake efficiency and lower emissions of CO2.
At all speeds and loads tested, ethanol exhibited no tendency to knock and could be operated with MBT (Minimum advance for Best Torque) ignition timing (unless constrained by cylinder peak pressure).
At target torque levels representative of a boosted gasoline engine, ethanol operated with lower boost pressure and MBT ignition timing, with lower exhaust temperature, higher efficiency, and lower emissions of CO2. Also resulting were an increase in cylinder peak pressure and rate of pressure rise.
Engine output at speeds below 2000 rpm was significantly improved through optimization of inlet and exhaust valve timing.
At speeds of 1250 and 1500 rpm, ethanol exhibited output 300 kPa BMEP higher than that of gasoline.
For higher engine speeds typified at 4000 rpm, ethanol may operate at increasing load with minimal requirement for fuelling enrichment, and with ignition timing only constrained by cylinder peak pressure. On the other hand, gasoline operates at a high level of retard and requires significant enrichment to limit exhaust gas temperature, causing a significant loss of efficiency and constraining output.
At a start temperature of 25°C, ethanol and gasoline show no appreciable difference in starting or light-off performance as indicated by exhaust gas temperature.
The design and development of an engine intended for high output operation with ethanol should address requirements for higher compression ratio, higher peak cylinder pressure, faster rate of pressure rise, reduced incidence of pre-ignition and an injection system capable of high fuel flow rate.
Orbital is continuing its work in this area, with plans evaluate the effect of ethanol water content on high load performance; develop low temperature starting capability; and develop a turbo-charger application for transient performance and high specific output.
S Brewster. “Initial Development of a Turbo-charged Direct Injection E100 Combustion System” (SAE 2007-01-3625)