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Rethinking the combustion cycle

In spite of all the public talk of further perfecting diesel, hybrid, electric and fuel cell technologies, more than likely the future of automotive power lies in perfecting and evolving the gasoline-powered internal combustion engine, with emphasis on the combustion. This article from Just-Auto.com touches upon the current state of four research projects in that front. Three of them already had their own threads here in the Roundtable, and I'll provide those links at the end of each section. Here's the article:

FEATURE: Rethinking the combustion cycle
Source: Farah Alkhalisi
Electric vehicles and plug-in hybrids are in the headlines at the moment, but the traditional internal combustion engine isn't about to burn its last, writes Farah Alkhalisi. We talk to four consultancies about their research.

Its fundamental principles have been little changed since Karl Benz patented the first 'automobile' in the 1880s, though advanced electronic controls of valve timing, fuel injection developments and ever-more precise monitoring of combustion are all contributing to greater efficiency and lower emissions. Technologies such as cylinder deactivation, brake energy regeneration and stop-start systems also help that gallon of fuel go further, and turbocharging and supercharging - combined, in engines like Volkswagen's Twincharger units - enable downsizing to smaller, lighter powerplants.

Yet aside from a few minor diversions - Mazda's flirtation with the Miller Cycle in which the intake valves stay open for longer, the longer power stroke of the Atkinson Cycle as in the Toyota Prius and other hybrids - mainstream modern engine technology has largely remained faithful to Nicolaus Otto's four-stroke principle. Despite Mazda's persistence, Felix Wankel's rotary engine is still little more than a novelty, for a start.

Until now: there are a number of interesting programmes underway, many in the UK, to completely rethink the combustion cycle. We talk to four consultancies about their research.

Ricardo's 2/4SIGHT technology enables a radical downsizing: its first prototype is a 2.1-litre V6 said to deliver performance on a par with that of 3.5-litre V8. "It combines a number of technologies already in the marketplace in an unusual way", says senior project engineer Richard Osborne, who describes it as "an evolutionary step in advancing gasoline engines, not a revolutionary technology."

The 2/4SIGHT unit features an advanced turbocharger and supercharger boost system - similar to Volkswagen's Twincharger technology - plus direct injection and a cam profile-switching system similar to Honda's iVTEC and Mitsubishi's MIVEC, but it can operate in both two- and four-stroke cycles. The two-stroke cycle "allows a very high degree of torque at low engine speeds", says Osborne, and overall fuel economy improvements are in a range of 25-30% depending on the vehicle application.

"Switching between two- and four-stroke cycles is completely novel, and achieving those switching moments without a negative perception by the driver is a main focus", explains Osborne, who adds that other challenges include controlling the variable valve-train, the boost systems and overall engine management and systems integration. A lean-NOX trap (LNT) is also required as part of the catalyst system, but the engine is Euro 6-capable without any difficulties.

Ricardo reckons that the 2/4SIGHT engine is cost-competitive with other advanced petrol engines, and cheaper to develop than an equivalent diesel. The first logical application is in a premium vehicle, says Osborne, "as this is the segment that has a problem with CO2", but "it would also suit slightly smaller cars and SUVs; it would go less well in very small cars because it is difficult to sustain the cost savings. It is less likely to be useful in a hybrid, where the engine is not running all the time, but there are still benefits and we can't eliminate it completely".

The technology still is five to ten years off production still, according to Osborne. "We're only one year into our demonstrator vehicle programme, and at the design stage for this particular vehicle." Jaguar is a partner in the project, though Ricardo has discussed the research work with all its clients.

(see: 330 lb-ft of torque from a 2-liter, 4-cylinder engine? )

The Scuderi Group is less reticent in its claims for its 'split-cycle' engine. "Automotive engines have basically run on the same piston configuration since the inception of the Otto Cycle about 130 years ago", says Vice President and patent attorney Stephen Scuderi. "This technology has the potential to be the most significant advancement in internal combustion engines since then."

"It's basically a new way pistons work that, because of the late firing of the spark plug, produces cleaner, more efficient combustion. In more technical terms, the Scuderi Engine is a split-cycle design that divides the four strokes of a standard engine over a paired combination of one intake/compression cylinder and one power/exhaust cylinder." The spark plugs fire 'after top dead centre' with combustion in one cylinder and air compression in the other, all in one revolution of the crankshaft.

In a naturally-aspirated engine, efficiency gains are 5-10%, but 15-20% in a turbocharged unit. With the addition of an Air Hybrid module - an air tank which collects energy otherwise lost in the combustion process - "we see gains potentially being 25-50%," says Scuderi, who adds that "we are also seeing torque levels in the gasoline engine similar to conventional diesel engines on the road today." Nitrous oxide emissions are reduced by up to 80%, and carbon dioxide output is lessened in line with the fuel efficiency gains.

Stephen Scuderi says that "we are in the final stages of testing now on our naturally aspirated gasoline prototype", and that the company expects the technology to reach production in the next three to five years. "We are currently in discussions - at varying levels - with 14 of the world's 20 top engine manufacturers… we anticipate our first licensing agreement to occur in the next year." The technology is "suitable for any piston-driven application", he says, including engines for the marine, aircraft and rail industries as well as for cars and power generators.

Scuderi is consulting with the Southwest Research Institute in San Antonio, Texas, with British firm Powertrain Technology Ltd an "exclusive subcontractor, responsible for a substantial amount of the engineering that has taken place to date." Shoreham-based PTL is currently developing the air-hybrid and variable valve actuation technologies.

(see: Scuderi unveils cut-away of split cycle engine
The lowdown on the Scuderi Air-Hybrid Engine )

Ilmor Engineering Ltd of Northamptonshire is probably best-known for its Formula One and IndyCar engines - but its 'five-stroke' prototype, a 700cc unit developing over 130hp, has much potential for economy-oriented vehicles.

The Ilmor engine "is a conventional four-stroke with an additional exhaust gas expansion cylinder which realises more energy from each combustion event", explains engineering manager Steve O'Connor. Fuel economy gains are typically 10%, but the cycle promises simplicity and reliability, as well as being relatively low-cost, and it allows for significant downsizing due to its power density.

"The engine is probably most suited as a range-extender for a hybrid vehicle," says O'Connor. "it is most efficient at high load and so offers significant advantages over conventional engines in this application."

The next phase of the programme, in which the engine is fitted into a vehicle, will add direct injection and a new turbocharger; "we also have some developments that will improve the transient response (driveability) of the engine, and the very light load fuel economy", says O'Connor. The second prototype will also be "around 20% lighter, more compact, and tailored to the type of vehicle it is put into." Ilmor is "in discussions with several OEMs about the engine, and we look forward to seeing it running in a vehicle in the not too distant future."

(see: Ilmor's 3-cylinder, 5-stroke concept engine adds to a rumored 3-cylinder future )

Lotus Engineering calls its single-cylinder engine concept Omnivore, because it can potentially use almost any alcohol-based fuel: petrol, ethanol, methanol or even a mixture. Omnivore combines a two-stroke cycle with variable compression, made possible by homogenous charge compression ignition (HCCI), explains chief engineer Jamie Turner. As HCCI gives flameless ignition, nitrous oxide emissions are lowered, and the concept also shows no emissions of unburnt oil.

A large 'puck' in the cylinder head moves to vary the compression ratio from 10:1 to as high as 40:1. "In a nutshell, we squash the charge until it goes bang", says Turner. Continuously-variable control of the exhaust valves means that poppet valves are not needed, enabling a monoblock construction without a head gasket - and making variable compression much easier. "No-one has been here before," says Turner.

Efficiency gains seen so far are 10-15% better than "the best current four-stroke", and HCCI allows for greater control of the load and engine speeds across a far wider range. Lotus' charge-trapping valve system aids residual gas control, and "provides a smooth torque curve like a four-stroke".

Lotus is in talks with a couple of manufacturers over the technology. It'd be well-suited to a number of applications - cars, bikes, planes and trains, for a start, says Turner - but it is still a good five years from production.

Turner feels that there is a lot of entrenched prejudice about two-stroke engines. "The reason for the dominance of the four-stroke cycle is, I believe, entirely historical - it has the weight of development behind it because it was easier to understand at the dawn of the internal combustion age. In comparison, the two-stroke cycle was too clever by half - all those processes having to happen simultaneously." Modern developments such as direct injection and HCCI fix the problem of emissions, but "the prejudice has blinded people to the inherent thermodynamic advantages of the two-stroke cycle."

Like Ricardo's 2/4Sight, Omnivore is a fresh look at existing - and even old - technologies in a new combination. Reinventing the engine in the 21st century may be less about revolutionary developments than evolving systems already familiar.

FEATURE: Rethinking the combustion cycle: Automotive News & Comment

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