Hyundai has revealed an ultra-efficient diesel engine, and although it’s a compression-ignition internal combustion engine — gasoline is its fuel. It has a common-rail fuel system, uses both a supercharger and turbocharger, and has diesel-like high compression. What it does not have is a spark plug or even a glow plug. According to Hyundai’s Nayan Engineer, “What we have is an ultra-efficient IC engine, a diesel compression-ignition engine running on gasoline.” What’s at stake is creating internal combustion engines that are clean, efficient, powerful and cost-efficient engines for the next 10 to 30 years.
The “gas in diesel” engine is a technology demonstration and collaboration between Hyundai Motors, Delphi, Wisconsin Engine Research Consultants (WERC), the University of Wisconsin-Madison (UW), and Wayne State University (WSU). Funding is by the U.S. Department of Energy and partners. Government funding amounts to $7.48-million over four years and the partners deliver equal value to the project.
Let’s get back to basics. Diesel cycle engines auto-ignite their fuel mix when air is squeezed hard, a compression ratio typically above 13:1, and fuel is injected a few degrees before top dead center. Spark-ignition Otto-cycle engines don’t compress as highly (until recently, 10.5:1 was high compression for production spark engines) and ignite fuel with a spark. With the addition of direct injection, Otto-cycle engines moved a bit closer to diesel technology. Differences included the complexity and expense of diesel’s high-pressure ignition system. Then there’s the turbos. While gas engines have toyed with turbos, and in the last decade about 15% of engines passenger car engines became turbocharged, modern diesels always use at least one, and often two turbos or an expensive variable-geometry turbocharger. You can see turbochargers gaining use as smaller engines nabbed the turbo advantage, then direct injection, to make a compact gas engine punch far above its weight. Ford’s EcoBoost engines are a great example, as are Hyundai’s GDI motors.
Over the last few decades a technology called HCCI or Homogeneous Charged Compression Ignition has been tried. With modern computers the required conditions can occur, but are limited to a narrow power band and rpm range. It’s an attractive technology because it offers lean burn for emissions and low fuel consumption, but is almost impossible to make work in the real world. Enter the modern Otto-Diesel engine.
Hyundai’s GDCI Gasoline Diesel Compression Ignition engine, unlike other IC engines, requires a very quiet in-cylinder environment; there’s no tumble and no swirl, which is absolutely opposite everything that’s been done in gas engines for decades. There, gasses are mixed in the cylinder before spark by tumbling and swirling the gas and fuel molecules to get a better, leaner and more homogeneous, and therefore better-burning mixture. Instead, Hyundai’s injection is very late, later than diesel, where fuel is injected earlier, and ignition takes place after TDC and the mixture is actually stratified in a partially premixed fuel environment.
Why this difference from SI or diesel motors? According to Engineer, “Gasoline doesn’t want to auto-ignite, diesel fuel does. Gas is locally slow (burning), locally lean.” The Hyundai chief says that to make this engine work there are several critical technologies, all enabled by modern turbos, injectors, and above all, modern engine computers.
“The keys are in the injection system: injectors are centrally mounted and use multiple late injections so that all the heat goes downstream,” he says. In other words, no fuel goes in until after TDC when a very small amount of fuel is injected to start a heat source (like diesel’s auto-ignition) and later fuel injection delivers something for the developing flame to consume and expand.
Another critical element beyond the Delphi injection system—did we mention it’s equipped with their Stop-Start — are the ePhasers that provide completely variable valve timing. Without that ability to move gasses in and out as necessary, the engine wouldn’t be possible. And variable valve timing contributes greatly to the low NOx and particulate emissions of the motor.
“The engine pressures are sub 500 bar with 14.8:1 compression ratio so there’s no pre-ignition or spark knock,” Engineer told Diesel World. So the compression is comparable to many modern diesels from BMW, Mazda, even Ford and Chevrolet. The days of 18:1 are as over as Courtney Love.
“There are no sparks, no flames,” he explains. “Boost comes from a turbocharger and a supercharger. We need that energy in the exhaust which equals heat, so the supercharger is needed at low-end rpm as a torque enabler.” Get it? At low rpm or low temperature, the supercharger puts energy into the cylinders to assist diesel cycle ignition. Oh, there’s more, much more. The engine’s fully variable valve train is run by electric motors, not chains or belts, and ePhasers are electronically computer controlled with lots of engine re-breathing, and the engine needs temperature to preheat the fuel. Yes, it’s a really fancy valve train and an engine with interesting complexities.
At Hyundai America Technical Center, Inc. the functional prototype on display was a dyno queen. Engines are built off production Hyundai Theta engine blocks capable of 180-200 bar pressures, but enhanced and strengthened. (US Theta II GDI engines displace 2.0 and 2.4 liters.) We suspect normal O-ringing of the head and advanced gasket technology is in use. The engine we saw has a dedicated aluminum head similar to a production Hyundai motor and incorporates splay valves and a standard 200-bar fuel pump. The supercharger is from Eaton and similar to the one on a Corvette. Once the engine is running it hands off to the turbocharger. Other necessary technologies include a low-pressure EGR loop with the engine running up to 30 to 40 percent EGR, which is very, very high. Another contributor is that Phase 2 engines have adopted more sophisticated diesel-like piston design.
In every phase of development, the team has also focused on friction reduction. This engine accomplishes many tiny efficiency gains, like 1 percent from a rollerized camshaft, 3 percent from rollerizing the cranktrain, 1 to 2 percent by recovering waste heat. They’ve optimized the two-stage oil pump and lowered the rpm limit to 5,000, coated piston rings with low-tension oil control rings, and coated the piston skirts. Even cooling the EGR offers a 3 percent improvement in efficiency.
With the torque the engine is expected to generate, engines can be smaller. Smaller engines with a down-speeded transmission and higher gears will result in greater fuel economy. Hyundai and partners have targeted 25 percent better fuel economy for a given engine size or family and reports published in 2012 show 13 percent combined city/highway improvement. It does require a three-way catalyst and diesel oxygen catalyst but no NOx catalyst. Claims for the engine include very low NOx emissions and no smoke on 87-octane fuel.
While the engine we saw has been tested, Hyundai says it hasn’t been tested under transient driving conditions, the kind your engine experiences every time you lift off the throttle or jam it to the floor. So, for now the engine runs at 4,800 rpm with externally sourced hot EGR used when the engine is cool. However, DOE reports do show running prototypes, so there must be tests in the wild, though perhaps not of this particular build.
Engineers say they continue to work on reliably igniting fuel at temperatures below -40°F “as we will not have spark plugs.” Oh, and the engine sounds like a diesel, complete with solenoid injectors that are used for cost savings. While we think piezoelectric injectors may find their way into the mill, they’re expensive and with the already complex extras required, every bit of cost savings is important. Factor in that solenoid injectors are now as capable as piezoelectric injectors of just a few years ago and maybe we’re way off base in that prediction. And if you’re wondering why all the bother to create a new class of engine you need look no further than the emissions regulations set to phase in next year. We’re betting we will see similar technology for small engines emerging from many automakers. For one, it makes sense, and we know clean-burning engines are needed globally as well as at home. This research also opens whole new avenues in diesel engine technology, and we’re darn thankful. DW
Hyundai is not the only company working on gasoline fuel use in a diesel engine environment. We’ve also seen early examples of a similar setup from Audi and Aragon National Labs. In both cases, the goal is to reduce the emissions in a gasoline-burning engine. As technology marches on, we’ll see more of these hybrid engines and most likely other new technologies as well.