Scania has backed the development of a new diesel engine, promising a more efficient, cleaner and cheaper compression ignition option
There is a more efficient, cleaner, cheaper diesel engine on the horizon and bus operators will be the major beneficiaries when it comes to market.
Called Homogenous Charge Compression Ignition (HCCI), the new approach to diesel promises to bring a big step forward in fuel efficiency and emissions at a time when the conventional diesel engine has just about reached its limits.
“The reality is that the diesel engines we see on the road today are edging very close to the theoretical maximum efficiency of a diesel engine,” Scania Commercial and Technical Advisor Graeme Miller, pictured, told the recent BusVic Maintenance conference.
“We have only got a few percent to go,” he says.
Miller says the only way to achieve significant gains in fuel efficiency and emissions in future is to perfect a system like HCCI, which he describes as the ‘Holy Grail’ of diesel engine design.
The main idea behind HCCI is to more accurately control the combustion process.
“Timing is everything, as they say, and it is easy to achieve with a petrol engine because you can initiate a spark whenever you like,” Miller says.
“With diesel, the timing of combustion has been controlled through the fuel injection system and, while this gives some accuracy, it does not give good mixing of the air and fuel in the cylinder.
“This means it starts igniting before the air and fuel is evenly mixed, promoting a flame front that moves from the richest to the weakest part of the mixture.”
As its name suggests, the key to the HCCI engine is a thorough mixing of the air, fuel and re-circulated exhaust gases.
“We get the air, fuel and EGR all premixed before we put it into the cylinder, just like you do with a petrol engine,” Miller says.
“Then what happens is we have the compression ignition and everything ignites together and we have our exhaust stroke.”
The timing can be further influenced by the use of variable valve timing, so the charge does not reach ignition until the right moment, and there are also projects looking at variable compression ratios and variable-vane turbo-chargers.
Miller says that, if everything ignites at the one time, all the fuel is burned and none is blown out the exhaust pipe, as happens now.
“The result of that is we have a very highly efficient engine. It is very, very fuel efficient, much more efficient than the current crop of diesel engines.”
Less fuel means less CO2, but there are other emissions gains, according to Miller.
“It also is what we would call cold combustion, and the reason we are looking for cold combustion is that NOx is produced at high temperatures.
“With this relatively cooler combustion, we have reduced NOx to very, very low levels – almost undetectable – and also, because it is such an efficient burn, we also get less carbon or particulate matter produced.”
Miller also had a word of consolation for operators who might have been frustrated that HCCI will not be market-ready for ‘a few years’.
He says one of the most important factors in fuel reduction is driver training.
“We have seen in tests which include similar traffic and atmospheric conditions that there can be a 20 to 30 per cent difference in fuel consumption from the best to the worst driver.
“The driver is probably the biggest single biggest influence on fuel costs.”