Folke Fritzson has committed himself to gas engine development since the mid-1990s. Now, with the renewed interest in gas power, he and his team at the Scania Technical Centre have their eyes on the next generation of gas engines.
Gas engines work according to the Otto principle, relying on spark ignition as opposed to the compression ignition used in diesel engines. A significant advantage of an Otto engine is the low noise level, much quieter than a diesel engine. In addition, thanks to the lower compression ratio and combustion pressure in the cylinder, the strain on engine components is considerably lower, making for a long service life.
Being inherently clean, up until the introduction of Euro 3, gas engines only needed noise certification to become homologated. They managed to meet both Euro 3 and Euro 4 exhaust emission levels with flying colours.
The lean-burn principle was chosen for all gas engines before Euro 6. Lean-burn involves the mixing of gas into the intake air before it enters the cylinders. When Euro 5 was due, the six-cylinder 9-litre engine was replaced by a modularised five-cylinder version of the new 13-litre six-cylinder diesel engine. The swept volume increased from 8.5 to 9.3 litres, enabling power and torque to be raised for better performance.
Exceeded the standards
From Euro 5 in 2009, a comprehensive test procedure applied to all heavy-duty engines, including those running on gas. The Euro 5 gas engines exceeded the standards and most were certified according to the EEV (enhanced environmentally friendly vehicle) standard, which is slightly stricter than Euro 5.
The team continued to refine the design in preparation for the much stricter Euro 6 regulations. Fundamentally the engine had excellent breathing. The areas needing attention were the intake tract, the gas injection and the exhaust after-treatment system.
Individual gas injectors
The intake now accommodates individual gas injectors, located in the intake manifold, close to the entry into each cylinder. This provides stoichiometric combustion, where exactly the appropriate amount of fuel is injected at any time; there is neither an excess of air nor gas. Exhaust gas recirculation is used to reduce the combustion temperature – exhaust gases diluting the intake air will lower the oxygen content and reduce the combustion temperature.
The small turbo maximises engine response, which is a most important Scania trait that contributes strongly to driveability. The after-treatment system features a three-way catalyst to cater for each of the three legislated substances: NOx (nitrogen oxides), HC (hydrocarbons) and CO (carbon monoxide).
Performance similar to diesel
The progress made over the years is such that performance is now similar to Scania’s corresponding diesel engines, while fuel economy and efficiency are better than ever, despite the drastic cut in exhaust emissions.
Actually, thanks to the advanced engine management system, Scania’s Euro 6 gas engines undercut all certification limits by a wide margin. The Euro 6 engines are also much more forgiving than their predecessors when it comes to gas quality.
“Being a renewable fuel, biogas provides an excellent low-carbon alternative,” says Fritzson. “With high-quality biogas, CO2 emissions drop by up to 90 percent compared with regular diesel fuel.
“Occasionally insufficient biogas supply forces operators to run on a mix of biogas and fossil natural gas. The resulting quality may be a bit inconsistent. However, Scania’s engine management system has been designed to cope with methane variations in the fuel of up to 40 percent without adverse effects on driveabililty.”
Bus and truck operators show high interest
There is no denying the pride of the design team, headed by Peter Daelander, with Fritzson as the key driver for the third successive generation of Scania gas engines.
“It’s great to see sales of the Euro 6 version took off directly, with more than 250 gas vehicles sold during the first six months after the launch,” says Daelander. “The interest is equally high among bus and truck operators. Greater volumes mean we will get access to even better resources for our research into renewable fuels and alternative propulsion.”
Scania’s Gas Engine Timeline
Lack of petroleum during the war means local fuels have to be used to maintain a basic transport system. Burners are fitted to the vehicles, where wood or charcoal is burned to produce methane gas, make producer gas partly containing which is then combusted in the engine.
Scania supplies a couple of thousand “long blocks” (incomplete engines) ready for gasification as well as diesel engines to US-based genset supplier Waukesha. Scania also starts to supply long blocks for gasification to other manufacturers.
Scania’s gasification of the 11-litre six-cylinder engine gets off to a great start with a deal for more than 200 gas-fuelled long-distance coaches to Australia. Engines are also adapted for trucks, primarily for waste removal. The gas bus market starts to gain momentum thanks to the advantageous emission characteristics. Scania supplies a couple of thousand units, including a large batch of Euro 3 and 4 engines for city buses.
In time for Euro 3 in 2001, Scania’s team finalises the design on a new gas engine, this time based on the six-cylinder 9-litre engine. Performance is identical to the 11-litre it replaces, and it fits neatly longitudinally or transversely in city buses.
Euro 5 (from 2009) and subsequently Euro 6 (from 2014) involves a major technological step, with advanced interaction via a CAN-bus between the engine management system and other control systems on the vehicles.
With its Euro 6 engine range, Scania establishes a market lead in alternative fuels. The two new Euro 6 gas engines are compatible with the cleanest road fuel available – biogas – or any mix of biogas and natural gas. The gas engines can be operated on biogas or compressed natural gas (CNG). Both gas types can be liquefied, which makes on-board storage much more compact.