German Car Zone

The Magic of DiesOtto and HCCI


Imagine a gasoline engine that runs with the efficiency of a diesel, only with no soot, no rattle and no compromise in power.

OK, you can stop imagining, because it's already on the way. The Mercedes-Benz F 700 introduced at the 2007 Frankfurt Auto Show has brought the future within reach and it's called DiesOtto, a name so clumsy that only a German engineer could have thought of it.

It's a concept so brilliantly simple that it sounds like one of those pills you drop into your gas tank that's supposed to get you automatically better gas mileage, an unlikely combination of chemistry and animal magnetism.

The DiesOtto Idea

Twenty years ago, a 1.8-liter gasoline engine typically delivered 115 horsepower smoothly and quietly, while the best you could expect from a similarly sized turbocharged diesel would be 90 hp and a splitting headache. Over the past two decades, the diesel engine has been developed to deliver 20 percent more power and 60 percent more torque (largely thanks to turbocharging), plus cleaner air emissions and far greater refinement. Advances in gasoline engines over the same period have been comparatively meager, with a peak power increase of perhaps 15 percent.

But all this is about to change.

At the Frankfurt auto show in September, Mercedes-Benz pulled the wraps off the F 700, a research vehicle that's meant to demonstrate technologies that might be incorporated into the S-Class sedan of the future. The key item is an engine called DiesOtto, honoring Rudolf Diesel, the inventor of the compression-ignition internal combustion engine, and Nikolaus Otto, who popularized the four-cycle internal combustion engine.

Mercedes-Benz's DiesOtto is two engines in one, the old two-for-one concept that has everlasting appeal to engineers of all persuasions. It is a spark-ignition Otto-cycle engine that runs on gasoline, yet it also has the capability to transition into diesel-type compression ignition for improved efficiency.

The key is the new combustion strategy known as Homogenous Charge Compression Ignition (HCCI).

HCCI Is Our Friend

In a gasoline engine, a mixture of fuel and air is drawn into the cylinder, compressed by the rising piston to something like a tenth of its initial volume, and then ignited by a spark from the spark plug. For part-load operation — which means most of the time — a throttle valve in the intake is partially closed to reduce the amount of air reaching the cylinders. Because the engine wastes power by sucking air-fuel mixture into its cylinders against the restriction of the throttle, its efficiency under partial load is relatively poor.

Because a diesel doesn't require a throttle, it has much better efficiency under partial loads. As air is compressed in the combustion chamber, its temperature rises. As the compression ratio reaches 20:1 or more, the temperature is great enough to spontaneously ignite the diesel fuel that is injected into the combustion chamber. The diesel engine is governed by metering the amount of fuel injected instead of timing the ignition spark.

At the same time, a diesel engine has its drawbacks. Because it has to be strong enough to withstand the vibration caused by combustion under such extreme compression ratios, it is heavy and expensive to manufacture. And there is the cost of the after-treatment technology needed to clean the exhaust gases.

For some time, researchers at every major car company have been investigating homogeneous HCCI in spark-ignition gasoline engines. It combines the technique of spark ignition when the engine is cold or accelerating with the principle of diesel-style auto-ignition under partial loads.

Under partial loads, a comparatively lean air-fuel mixture is allowed to compress in the cylinder until the temperature and pressure rise high enough to ignite the mixture. In gasoline engines, this sort of uncontrolled pre-ignition can be bad news, because the intention is to control combustion timing using the spark plug. But HCCI uses this spontaneous ignition to produce a flameless release of energy throughout the entire combustion chamber, which promotes complete combustion and relatively low temperatures.

Lean operation helps the HCCI engine approach the fuel efficiency of diesel, while clean, low-temperature combustion doesn't require elaborate exhaust treatment to meet air emissions standards.

Caution: Cleverness Required

The clever bit is to control combustion timing by managing conditions inside the combustion chamber, so you avoid the uncontrolled detonation (which we hear as "knocking").

Professor Uwe Grebe, executive director for GM Powertrain Advanced Engineering, says, "It's all about measuring the pressure in the cylinder and determining what needs to be changed in terms of valve timing and in terms of injection so you achieve the optimum timing for the auto-ignition." GM built an HCCI version of its 2.2-liter Ecotec inline-4 that produces 180 hp and 170 pound-feet of torque, then fitted prototypes to the Saturn Aura and Opel Vectra, which American journalists were invited to drive in Detroit last summer.

Exhaust valve timing turns out to be particularly important. By simply closing the exhaust valve early, the exhaust gas trapped in the cylinder helps to heat up the incoming charge sufficiently to produce auto-ignition. GM calls this process "recompression," because the remaining gas is compressed by the piston as it continues to rise. Mercedes-Benz uses the same technique for its DiesOtto HCCI engine, but calls it "internal exhaust gas recirculation."

Even More Cleverness

Another way of ensuring the gas in the cylinder reaches auto-ignition temperature is to physically raise the compression ratio.

"HCCI is all about auto-igniting the mixture in the cylinder, so you need to exceed a certain temperature," notes GM's Grebe. "You can do this by mixing exhaust gas and fresh gas and generating a mixture temperature, or by compressing it harder by changing compression ratio."

Though GM says a system for variable compression ratio is too complex and costly for the potential benefit that is produced, Mercedes-Benz opted to incorporate such a system into DiesOtto. It's the important technology that helps make HCCI practical in the Mercedes F 700's DiesOtto engine.

German Engineering

As featured in the F 700, the Mercedes-Benz DiesOtto engine combines a small gasoline engine, two-stage turbocharging, direct fuel injection, variable valve timing and HCCI, which Mercedes calls controlled auto ignition (CAI).

Though Mercedes is slightly coy about the details, it admits that it achieves variable compression by physically influencing the crankshaft rather than manipulating the cylinder head. Similar systems mount the crankshaft in eccentric main bearings and then rotate the bearings to change the height of the crankshaft in the block, and it's likely that Mercedes has done something similar.

DiesOtto's project leader Günter Karl says, "We can adapt the compression ratio very well to the operating points, so we have good efficiency." He continues, "Under part load we go as high as possible — higher than spark ignition but lower than diesel. At wide-open throttle we can go lower, so we have no knocking."

The DiesOtto's two-stage turbocharger system is very like that of a diesel engine, as a small turbo spools up quickly for boost at low rpm, while a larger turbo produces enough volume to sustain boost at high speeds.

The F 700 Puts HCCI on the Road

As featured in the Mercedes-Benz F 700, the 1.8-liter inline-4 develops 238 hp. In this application, the DiesOtto engine is supplemented by a 20-hp electric motor that engages during acceleration away from a stop. The drivetrain's total torque output is rated at 295 lb-ft, the equivalent of 162 lb-ft per liter. Mercedes tells us this compares to 133 lb-ft for comparable-displacement turbodiesels, 111 lb-ft for turbo gasoline engines and 74 lb-ft for normally aspirated gasoline engines.

The drivetrain is capable of thrusting the 3,748-pound F 700 to 100 km/h (62 mph) in 7.5 seconds, while the car's top speed is electronically limited to 120 mph. The engine's fuel efficiency plus the use of the electric motor as a stop/start mechanism (which makes the F 700 a mild hybrid) enables the car to achieve 44 mpg on the European test cycle, accompanied by very low CO2 emissions.

Coming Soon?

All this sounds enormously promising. But as Mercedes-Benz engineers tell us, there are hurdles to overcome before DiesOtto hits the showroom. Günter Karl says, "We have a pressure transducer in each cylinder for closed-loop combustion control. This is absolutely necessary to get good transient behavior between HCCI mode and spark-ignition mode." And that transducer is currently very expensive.

Meanwhile, research continues into optimizing electronic control of this complicated engine. "The big challenge is the high number of variables," explains Karl. "With a system this complex, it's impossible to handle it only using [electronic] maps. You need real physics in the ECU."

So how long before we can buy DiesOtto? "Some of these features you will see soon in series production, but the whole combination will take some time," says Karl. "My boss says, 'In the midterm.' Whatever that means."

Meanwhile, GM executives are also enthusiastic at the prospect of an HCCI engine because it promises diesel efficiency without the cost of diesel manufacture and diesel emissions treatment.

But reckon on seven years before diesel economy and torque are matched to the clean emissions and broad power band of a gasoline engine in a car you can buy. If the results match the promise, it will be worth the wait.


Green Speed: Diesel Power Is Coming to Your Gas Engine


M

Very interesting stuff. My friend is researching into HCCI technology for his thesis project. There is a single cylinder test engine in our engine lab that can be run on diesel or otto cycle, but it can't run a combine diesOtto cycle though. The test engine also has a mechanism to raise or lower the crankshaft position relative to the engine head and block to increase or decrease the compression ratio of the engine.

Excellent news,that's very promising high tech.Just wish it doesn't come late