Small engines, big future?
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Would you accept a smaller engine in your next truck? We examine the case for engine downsizing in the not-too-distant future Words: David Wilcox Volkswagen offers a 1.4-litre petrol engine, developing 120hp, in the big Passat saloon. Ford's threecylinder, 123hp, 1-litre EcoBoost engine sits nicely in a Focus. Engine downsizing is all the rage in the car business.
Driving this trend is fuel economy; car makers need to hit European CO2 reduction targets. There is a similar scenario in the van world, now also working towards CO2 reduction targets. For example, Ford has rationalised Transit diesel engine choice to multiple ratings of a 2.2-litre unit, axing the previous 2.4-litre engine. And all Vauxhall Movano/Renault Master models now use various ratings of a 2.3-litre engine diesel, replacing the former 2.5-litre engine. In both cases, downsizing means better fuel consumption but no less power or torque.
Draft European proposals for truck CO2 reduction targets are expected this year, so is engine downsizing coming our way too? Looking at Euro-6 truck engines waiting in the wings, far from downsizing, the trend appears to be upsizing. For instance, the new Renault/ Volvo Euro-6 engines for middleweight trucks have swept volumes of 5.1 and 7.7 litres, up from the current 4.8 and 7.2 litres. Iveco, too, is increasing the swept volume of its Tectors and two of its three Cursor engines. MercedesBenz's Euro-6 engines for its middleweights are at 5.1 and 7.7 litres, up from Euro-5's 4.3, 4.8, 6.4 and 7.2 litres.
However, downsizing looks more likely for tractor units and heavy rigid chassis, with established 13-litre engines giving ground to 11-litre alternatives. Volvo/Renault's 10.8-litre unit, MAN's 10.5-litre D20 and Iveco's Cursor 10 have already made in roads and they are joined at Euro-6 by two important newcomers: Daf's 10.8-litre MX-11 and Mercedes' 10.7-litre OM 470. The MX-11 is available in four nominal ratings up to 440hp/2,100Nm and Daf expects 80%-85% of its Euro-6 CF models to have this engine. Mercedes' new 10.7-litre OM 470 is offered in three nominal ratings up to 430hp/2,100Nm.
Both engines look spot-on for tractors grossing up to 40 tonnes or so, and for many multiwheel rigids too. The table (right) shows how well the top ratings of these two new 11-litre engines compare with the lowest ratings of the 13-litre equivalents. It is a fair guess that many more trucks will downsize from 13 to 11 litres in the next few years, unless running consistently at maximum weight on long-distance work.
Conservative attitudes about swept volume and fears of compromised durability are major stumbling blocks for downsizing. But those views are based on engines from the past; looking ahead, there are more reasons to suggest that smaller engines could become feasible and desirable.
Fuel economy "Downsizing is the next step in reducing CO2 emissions," says Daf Trucks chief engineer Ron Borsboom. These fuel savings stem from a reduction in an engine's frictional losses. About 50% of an engine's friction is at the piston/ liner interface so a smaller swept volume means lower frictional losses. Smaller pistons also have less weight to accelerate, further reducing power losses. The reduction in swept volume must not be offset by higher engine-speed — that would drive up frictional losses again.
Daf's MIX-i1 and Mercedes' OM 410 develop maximum power and torque at similar engine speeds as their 13-litre peers. Daf expects the MX-11 to be about 3% more fuel efficient than an MX-13 on similar work. And Renault Trucks says operators should choose its Euro-6 DTI 11 ahead of the DTI 13 if fuel consumption is a priority.
Weight Incorporating EGR coolers and exhaust after-treatment systems for Euro-6 adds at least 100kg to the weight of small rigid trucks and about 200kg to tractor units. Downsizing the engine can counteract this, clawing back payload capacity for weight-sensitive applications. The MX-11 is said to be 180kg lighter than an MX-13 and Mercedes' OM 470 is 140kg less than the 12.8-litre OM 471.
Thermal management Thermal management — keeping exhaust temperatures up — is going to be crucial at Euro-6. A cool exhaust accelerates the accumulation of unburnt sooty particulates in the diesel particulate filter (DPF). This raises exhaust backpressure, triggering active regeneration of the DPE This can be achieved in numerous ways, but all of them use more fuel. This would be avoided by opting for a less powerful engine working harder. Scania advises operators whose engines seldom run at full load to consider lower power ratings at Euro-6.
Thermal efficiency A big change on the horizon is boosting a diesel engine's thermal efficiency from 44%. Experts believe that 50% is attainable by 2020. Either waste heat recovery (WHR) systems or turbo-compounding, both capturing the energy in the exhaust, are key to this, said to be able to extract an extra 4%-5% of power from an engine. But to make either worthwhile, you need lots of energy in the exhaust: a smaller engine working harder fulfils this. It would also save weight and space, needed to accommodate the equipment: a WHR system weighs about 150kg, says Ricardo.
Aerodynamic savings Changes proposed by the EC in April will allow truck noses and trailer tails to be extended to permit more streamlined designs, with fuel savings approaching 15% at motorway speed. That suggests that power requirement at this speed for a typical artic would be cut by about 25hp.
Durability Similar engine speeds, with improved designs and materials, means downsized engines can offer acceptable durability. Mercedes says the B10 life (the point that 90% of engines should reach without major failure) of its OM 470 is 1.2million km, the same as the 12.8-litre OM 471. • REDUCED LOSSES Engineers have made great strides in reducing all manner of power losses in heavy-duty diesels. For example, Federal Mogul says its latest pistons slash total piston friction by 17%. To a similar end, Scania is using a new low-friction cylinderliner coating on its Euro-6 engines. Low-viscosity engine oils, such as 10W-30 grades that sap less power than traditional 15W-40 oils, are gaining traction in the market.
Attention is also focused on limiting power required to drive engine ancillaries. Expect to see more clutched air-compressors, totally disengaged when air-tanks are full, and smart battery-charging that reduces alternator load. Oiland waterpumps are also prime targets for more sophisticated management, reducing their power demands. Variable-speed pumps are one way of doing this, while Daf says better direction of the piston-cooling oil jets in its Euro-6 MX engine means the oil pump runs more slowly because it needs to pump less oil; a bigger radiator cuts cooling fan engagement by 50%. Longer term, an increasing number of ancillaries are likely to be electrically-driven. All these incremental savings mean more power is available to go down the driveline, so not quite so much is needed at the outset.
CYLINDER PRESSURES The key to extracting more output from a small engine without up-speeding is higher peak cylinder pressures (PCP). Better materials and superior designs for cylinder heads, pistons and bearings allow PCP to rise, from a maximum of about 150bar for typical tractor unit engines 10 years ago to as much as 220bar today, enabling smaller engines to do the work of bigger ones. It does not stop there; engineers are working on pistons and cylinder heads designed for 250bar.