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Diesel’s combination of refinement, driveability and fuel economy is what makes it Europe’s engine of preference. But it’s far from perfect. Despite advances in common rail injection and turbocharging, diesels produce more NOx than gasoline engines and lean NOx traps or selective catalytic reduction catalysts, together with diesel particulate filters, add to their cost.

Gasoline is fighting back. Engineers are working to bring homogeneous charge compression ignition (HCCI) on to the roads using regular fuel. The combustion process is complex, but promises to reduce gasoline engines’ CO2 and NOx and they could still cost less than a diesel.

HCCI should close the gap on fuel efficiency too, as its part-load performance is similar to that of diesel. The process uses spark ignition for starting and full load, switching to compression ignition, or controlled auto-ignition, at part-load.

“For lean mixtures or high EGR rates, maximum gas temperatures are low,” says Matthias Rühl, powertrain director at engineering firm Bertrandt. “At low loads, engine-out NOx emissions can be almost zero.” Auto-ignition avoids the temperature peaks that contribute to NOx formation. Soot emissions are also lower because combustion is homogeneous. Temperature and the point of ignition depend on compression ratio, initial gas temperature, residual exhaust gas and the air:fuel ratio.

Unlike diesel or stratified charge GDI, aftertreatment comprises a three-way catalyst only. But HCCI engines are, at best, prototypes. Much work is needed before the technology is ready for series production.

Bertrandt is working on HCCI for customer projects and says nearly all OEMs are developing similar new internal combustion processes. Even so, it doesn’t expect to see the technology on the market before 2012.

“Combustion control, in particular at transient operation, is one of the major challenges,” says Rühl. “The difference between misfire, desired fire and fire with unacceptable NVH in combination with increased mechanical stress is very small.”

Auto-ignition requires fast, accurate control of gas temperatures and mixtures. Rühl says a variable geometric compression ratio is one possibility, and expects between 14:1 and 18:1 for HCCI and about 11:1 for conventional gasoline operation.

DaimlerChrysler showcased its HCCI engine at Frankfurt in the F700 luxury concept car. Named Diesotto, the 1.8-litre unit with dual-stage boosting and variable compression ratio extracts 175kW/400Nm from four cylinders and meets Euro VI emissions regulations.

“The most challenging issue is putting all the technologies into one engine,” says Dr Leopold Mikulic, vice-president of powertrain development at Mercedes. “The system comprises modular components. Some we use today, such as GDI. Next-generation turbochargers will come quite soon, then the combustion system and variable compression ratio. We’ll see this some time beyond 2010.” The Mercedes engine is highly boosted and features a variable compression ratio.

The mechanism is built into the cranktrain, but other solutions are possible, depending on function, weight and integration into existing engines. Mikulic says compression ratios between 9:1 and 14:1 are likely, but could reach 15:1.

Reducing the degree of downsizing means variable compression could be avoided, but Mikulic doesn’t see this as the way forward. “By doing so, the area in the engine map where the system is working effectively is reduced,” he says. “To significantly improve overall performance you go for variable compression ratio.”

GM has prototype HCCI engines running in Opel Vectras. The 2.2-litre four-cylinder units are naturally aspirated, developing 134kW/230Nm. Compression ratio is fixed at 12.2:1.

“Variable compression isn’t needed in our engine. HCCI is all about igniting the mixture in the cylinder, so you need to exceed a certain temperature,” says Dr Uwe Grebe, executive director of advanced engineering at GM powertrain. “You can do this by mixing exhaust gas and fresh gas and generate a mixture temperature, or by compressing it harder by changing compression ratio.”

GM considered variable compression, but opted for a control of HCCI that doesn’t need it. Grebe sees no significant improvement by using it and doesn’t want the extra complexity.

His HCCI engine improves economy on the combined cycle by 15 per cent compared with the equivalent port-injected version.
“We’re better than stratified charge GDI,” says Grebe. “There you only get a slight improvement in fuel economy.

Turbocharging is something you can add to improve full-load performance and downsize the engine. This is the next addition.”

Thermal efficiency of HCCI engines can’t match diesels, but Mikulic says they can get close. Grebe says his engine gives 80 per cent of the efficiency increase of a diesel, but is less complex.

“Prior to boosting our HCCI is conventional, but with GDI it has fast cam phasing, a two-step valve control mechanism and closed-loop pressure sensing,” says Grebe. “Control is most important for HCCI, measuring cylinder pressures and determining changes to valve timing and injection to achieve optimum timing for the auto-ignition.”

The ECU reads pressure traces in each cylinder. Control algorithms adjust intake and exhaust cam phasing and injection timing. The complexity is huge. “For spark ignition it’s just about determining when the spark is needed,” says Grebe. “HCCI has three inter-related control mechanisms.”

Pressure sensing for Mercedes and GM will come from piezo-resistive units similar to the glowplug solution both OEMs will use soon in production diesel engines. Sensors could be integrated into spark plugs but, says Grebe, the high voltage would disturb output signals.

Precise control of EGR is also vital, using the EGR valve and fast-responding variable valve timing for cycle-to-cycle control of exhaust gas trapped in the cylinders. “Our EGR rates are much higher than a GDI turbo engine - more than 50–60 per cent,” says Mikulic.

With such advanced engine control, could engines operate in HCCI mode up to full load to maximise the efficiency benefits? Not yet, says Bertrandt. “Mechanical stress due to a very rapid combustion process would be excessive,” says Rühl. NVH would be a problem too.

GM agrees, adding that residual exhaust gas reduces the potential of cylinders to produce a full load, unless turbocharged. “The benefits are questionable when you expend so much energy compressing the gas,” says Grebe.

DaimlerChrysler thinks it possible in terms of basic thermodynamics, but having all parameters going the right way is a big challenge. “Maximum torque needs lots of fresh air,” says Mikulic. “It’s a question of the pressure ratios the turbochargers could deliver in order to have sufficient air supply.”

Full-load enrichment is another issue. Extra fuel is used in conventional turbo engines, harming efficiency. The compression ratio could be lowered instead, but this drops efficiency too. “You have to find a compromise between spark timing and compression ratio,” says Mikulic.

Cost should still be less than diesel. Engine construction doesn’t need to be as robust and measures such as cylinder pressure sensing are coming to diesels too. Aftertreatment should be considerably cheaper.

Mikulic says Diesotto will cost somewhere between a gasoline and diesel engine of equal torque rating. “The base engine is all-aluminium, so there’s also a weight advantage over a high-performance diesel,” he adds.

Bertrandt is again more cautious. “We expect the four-cylinder HCCI engine will be similar to the diesel engine, but development and calibration effort may be higher,” says Rühl.
GM and Mercedes make lots of luxury cars and SUVs, in which HCCI could significantly reduce CO2 averages. This might mean developing multi-cylinder applications, but the engineers want to keep things simple.

“Our main aim for now is the four-cylinder Diesotto engine,” says Mikulic, ”but we’ve started studying the implementing the variable compression system into V6s and V8s. It’s quite a challenge.”

Grebe too would prefer to stick with inline fours for GM’s first series application. “It can be done in any configuration, but the control system depends on how many cylinders you have,” he says. “If somebody gives me the task of putting it into production, I wouldn’t pick a V8.”