On the development of a low-emission propane engine for heavy-duty urban vehicle applications:Dissertation

At VTT, the development of stationary Valmet gas engines began in 1983. In 1988, a Valmet 311 engine equipped with a closed loop fuel system and a TWC was run on both natural gas and propane. The results were encouraging, and led to a decision to start the development of a heavy-duty vehicle engine. The study at hand describes the work beginning with the 311 engine up to vehicle applications with Valmet 612 engines. The next step was combustion chamber evaluation for propane using the 311 engine. Basically, two different combustion chamber types were evaluated. The final choice was made in favor of a high turbulence piston. A quantitative knock measurement system based on a rapid change of ignition timing and cylinder pressure analysis was developed. The experience from the three-cylinder Valmet 311 engine was then carried over to the six-cylinder Valmet 612 engine. With this engine, component temperature measurements, component evaluation and special exhaust emission measurements were carried out. The maximum efficiency of the naturally aspirated 612 LPG engine is 35 - 36 %. Piston temperatures should not cause any durability problems. The engine was also turbocharged to evaluate the possibilities of increasing the power output. The ignition system and spark plug type had very little effect on engine performance at lambda= 1. Lambda variations between individual cylinders could be reduced significantly by a small inlet manifold modification. A fuel control system allowing engine mapping was developed at Helsinki University of Technology. Ultimate emission performance using engine mapping, a modified inlet manifold and valve overlap was some 0.5 g CO and 0.1 g NOx/kWh in the ECE R49 test. A special two block catalyst was tested for improved dynamic emission performance. It was noted that ammonia is formed in the catalyst using a rich mixture. The Valmet 612 LPG engine has found its way into practical applications. Seven Sisu LPG trucks have been operational since 1993. The feedback in general has been good, although there have been some ignition and fuel system problems.