Abstract
Research and/or Engineering Questions/Objective
Although high compression ratios over 25:1 lead to heavy knocking phenomenon that results in loud noise, the high compression ratios over 25:1 and no heat loss to the chamber walls would theoretically achieve very high thermal efficiency of about 60%.
Methodology
The new compressive combustion concept based on supermulti-jets colliding with pulse, which was proposed by computer simulations in our previous reports (SAE papers & AIAA papers, 2011-2015), has a potential of high thermal efficiency over 50% even for engines having combustion chambers smaller than 100cc, because the supermulti-jets colliding at chamber center encase burned gas around the chamber center, which lead to complete air insulation. It is stressed that this supermulti-jets colliding at chamber center also brings encasing of combustion noise and also that single-point auto-ignition at chamber center can relatively accomplish silent combustion at very high compression, whereas traditional auto-ignition engines such as diesel one use rapid heat release due to multi-point ignition, leading to strong noise. Recently, we originally developed a small prototype engine system for gasoline, having a strongly-asymmetric double piston and the supermulti-jets colliding with pulse, although there are no poppet valves.
Results
This prototype engine, in which the number of jets colliding is eight while both of bore and stroke are about 40mm, can widely vary point compression strength due to the supermulti-jets and mechanical homogeneous compression level, by changing the phase of two gears between the double piston. For air-fuel ratios between 20:1 and 40:1 (lean conditions), steady-state experiments of combustion for this small gasoline engine having a medium level of point compression with an additional mechanical homogeneous compression ratio of about 8:1 show apparent increase of exhaust temperature over 100 degrees and pressures over 2.0 MPa, even at the situations without any plugs, while liquid fuel of gasoline is injected in intake-port (Injection pressure=0.35MPa), leading to homogeneous-charge of air-fuel mixture. It should also be emphasized that time histories of pressure after auto-ignition are similar to those for conventional SI gasoline engine, which implies silent auto-ignition of gasoline.
Limitations of this study
Although we confirmed efficiency of the new engine concept, there are still cyclic variations. Thus, we must try to test further in order to realize stable occurrence of auto-ignition.
What does the paper offer that is new in the field in comparison to other works of the author
Indicated thermal efficiencies calculated from the pressure histories are often larger than those for conventional SI gasoline engines with throttle valves, for some partial loads at engine speed of 2,000rpm. The present new silent engine without throttles will result in thermal efficiency comparable to that of Diesel one, while various fuels including gasoline can be used with cheap fuel injectors of low injection pressures setting in intake ports. Then, the present homogeneous-charge silent auto-ignition will also bring low emissions such as NOx comparable to those of conventional SI gasoline engines.
Conclusions
The combustion experiments clearly show the high potential of our new engine concept based on the supermulti-jets colliding with pulse. Silent auto-ignition of gasoline for a very small engine at very lean air-fuel ratios occurs, because of single-point ignition and also because of turbulent flame propagation at the later stage of heat release.
Key words: Engine, Thermal efficiency, Combustion, Fugine