Gas offers big advances in emissions: professor
BY IAN PORTER | 19th Jun 2009
Mixing lean-burn, low-temperature combustion with gas could improve the thermal efficiency of an engine from the petrol engine’s maximum energy efficiency of just over 30 per cent to more than 45 per cent, Prof Watson told a gaseous fuels seminar in Melbourne on Monday.
“Compressed natural gas burns faster and leaner than gasoline,” he said.
“With an octane rating of 140, natural gas can handle much higher compression ratios and deliver much higher efficiency.” High compression ratios of 15:1 or more could be used with technologies such as ultra-lean mixtures, the Miller cycle, jet ignition and direct injection to produce much cleaner and more efficient engines.
Prof Watson said lean burn usually promoted the production of oxides of nitrogen, an unwanted pollutant, but that this can be avoided by lowering the combustion temperature in the chamber and burning ultra lean.
“In order to maintain the power output, you need to run with high boost (from a turbocharger). If you add a lot of excess air, you can have lean burn at a low temperature,” he said.
This approach had already been proven by makers of gas-powered static diesel engines such as Wartsilla-Sulzer and Caterpillar.
Prof Watson said he had done a lean-burn project based on a 1996 Falcon more than 10 years ago, with good results. The Falcon had a compression ratio of 15.7:1 and a range of 450km on natural gas.
There was a 15 per cent improvement in thermal efficiency and a 35 per cent reduction in CO2 emissions compared with a petrol version.
He said more recent lean burn engines had employed the Miller cycle and direct injection to further improve efficiency.
A Miller cycle engine leaves the inlet valve open for the first 20 or 30 per cent of the compression stroke. This sees some of the new air charge expelled back out the inlet tract, but it delays the compression work, cutting the pumping losses all internal combustion engines suffer on the compression stroke.
The incoming charge of air is topped up by forced induction, which consumes less power to create than the crankshaft pressure. After combustion, the engine still gets full value from the fuel over the entire expansion stroke.
“We now have an opportunity to produce a gaseous fuel engine with half the CO2 of a gasoline engine,” Prof Watson said.
Current LPG engines can emit about 20 per cent fewer emissions than a petrol engine.
“What we have learned recently about managing the temperature profile in the combustion chamber has opened up a new opportunity for the industry in Australia,” he said.
“There are only two other countries in the world able to seize this opportunity.”
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