Clean start

Natural gas engine is claimed to efficiently produce the same amount of power as a diesel – with fewer emissions. Siobhan Wagner reports.

Natural gas engine with a high-energy ignition system is claimed to produce the same amount of power as a diesel engine — with fewer emissions — and it could help electricity and heat power generation plants comply with environmental regulations.

The engine from MAN, a Germany supplier of large-scale diesel engines, was first unveiled in 2006. The engine has since undergone a test trial with a Russian energy supplier and it recently received a big commercial order from Argentinean energy firm PAMPA.

Traditional gas engines use spark plugs, but this engine does not. Instead, it has a simple ignition system with high-pressure gas injection.

MAN calls the concept Performance Gas Injection (PGI), which it says generates ignition power up to 100,000 times greater than the most powerful spark plug.

The PGI principle allows natural gas to be converted with high degrees of efficiency into electrical and thermal energy with only minimal emissions.

One of the developers, MAN’s Wolfram Lausch, said the components involved have a much longer service life. ‘The system therefore forms the basis for an engine that combines the high-performance density and efficiency of a diesel engine with the advantages of a low-emissions gas model,’ he said.

Lausch and his colleague Günther Bozung conceived the PGI engine idea on a flight from Munich to Copenhagen in the late 1990s, while they were discussing how to include more gas engines in MAN’s portfolio. They were interested in those running on the four-stroke Otto cycle, which is used in internal combustion engines.

‘The problem was Otto gas engines were over-designed for handling the low mean effective pressures they run on,’ said Lausch. ‘Diesel engines are designed and structured to withstand high mean effective pressures, and it would have been too costly to convert diesels to Otto gas engines.’

The PGI concept allows the Otto gas engine to run at the same mean effective pressure as a diesel. The design of the engine — including the compression ratios — is similar to most modern four-stroke diesels.

On the first stroke of the piston, as the piston descends it reduces the pressure in the cylinder and a mixture of gas and air is forced into the cylinder through the inlet port. The inlet valve then closes and the following stroke compresses the air/gas mixture. Meanwhile, a small amount of gas is compressed to 300 bar in a separate gas valve.

The compressed gas is then injected into a pre-chamber through an open electrically-actuated, electronically-controlled valve. The gas hits the hot body of the chamber and ignites. This flame then ignites the air/gas mixture in the main combustion chamber. The pre-chamber contains a glow plug, which is an electrically heated wire that heats the combustion chambers and raises the air temperature at start-up. Thereafter the heat retained from the combustion process is enough to sustain ignition of the high-pressure gas.

One major technical problem Lausch and his colleague considered before testing the PGI concept was what would happen if the air/gas mixture pre-ignited during the compression stroke. If that occurred it could cause combustion misfiring and possible damage to the piston and combustion chamber components.

But Lausch was determined that their engine would overcome this tendency. With up to 100,000 times more ignition energy available than from a spark plug, the PGI engine is capable of igniting considerably leaner mixtures than spark-ignited gas engines so the air/gas mixture is less likely to pre-ignite.

According to MAN, the engine is ideal because it presents no new manufacturing challenges. ‘We can convert ourdiesels into Otto gas engines without increasing the cost,’ said Lausch.