How's Your Hcci?
How’s your HCCI?
TDC
Kevin Cameron
REMEMBER HONDA’S EXP-2 AFRICA rally bike? Its two-stroke engine employed a unique combustion system that operated over most of its load range without spark ignition. This type of combustion is now known as Homogeneous Charge Compression Ignition. Diesels employ compression ignition, but their charge-because fuel burns only as it is injected-is very non-homogeneous. Gasoline engines employ homogeneous (pre-mixed) charge, but ignite it by spark, not by the heat of compression. At the time the EXP-2 was first revealed in 1995, it was only a curiosity, but today HCCI is attracting a lot of interest.
Gasoline engines burn a chemically simple, highly refined fuel quite cleanly, but because detonation limits compression ratio, efficiency is limited. Operating lean, at part-throttle, and in a stratified-charge combustion mode, efficiency increases, which is why Japanese automakers are rapidly developing such engines. But in the U.S., where drivers can use more throttle, the efficiency advantage of these GDI (Gasoline Direct Injection) engines dwindles.
In Europe, where all petroleum must be imported and has always been heavily taxed, regulators decided long ago to favor low fuel consumption, giving diesel engines “softer” emissions standards and a fuel tax break to encourage their use. Currently 40 percent of new car sales in Western Europe are diesel-powered.
In the U.S., where exhaust emissions regulation originated in the need to eliminate serious smog in Los Angeles and other cities, mies for auto-sized diesel engines are strict enough to discourage their use. Diesel exhaust contains both nitrogen oxides (NOx) and particulates (soot).
Gasoline engines are the current darlings of the regulators because their light fuel burns more completely than the heavier diesel fuel, and it can be burned at a lean mixture, ensuring that there is plenty of oxygen to combine with all the fuel. Complete burning means low hydrocarbons (which include soot); lean combustion, which reduces flame temperature, is a primary means of preventing NOx formation. Unfortunately, as we know, gasoline engines are limited by detonation to compression ratios in the 10-12:1 range. The diesel engine’s 25 to 40 percent higher efficiency comes in large part from its ability to operate at compression ratios of 17:1 or higher.
But the diesel has a special problem because its fuel is injected after the piston has compressed an unthrottled charge of pure air. As the spray enters the combustion chamber, it is at first 100 percent fuel. As the spray breaks up, forming droplets, a very rich region is formed. Just ahead of the leading edge of the spray, there is 100 percent air. When the fuel ignites from contact with the hot compressed air, it does so in regions of mixture whose strength yields the greatest energy release. These are regions of chemically correct mixture-the mixture that traditional spark-ignition engines have burned since the beginning. Once the spray ignites, this region bums fast and hot.
Max energy release means max temperature, which forms NOx. Because of the nature of diesel injection sprays, there is no way to achieve a uniformly lean mixture that would burn cool enough to avoid most NOx formation.
Diesel combustion can be moderated by recirculating cooled exhaust gas to the intake. Unfortunately, by slowing combustion, this generates more soot. Choose your poison-either low soot with high NOx; or cut NOx with cooled EGR and accept higher soot. Engineers dream of diesel-like efficiency, but with a homogeneous charge like that of the gasoline engine.
The combustion system of the EXP-2 two-stroke depended upon retaining exhaust heat from the previous cycle by means of an exhaust gate. As the piston rose on compression, the heat of compression, added to retained exhaust heat, would push the homogeneous fuel-air mixture toward auto-ignition. At some point, the mixture would self-ignite at many points within its volume. It burned rapidly but without detonation. If combustion occurred too late in the cycle, the exhaust gate would close slightly, retaining more heat that would cause combustion to occur sooner-and vice-versa. Unfortunately, auto-ignition could not be maintained down to low throttle positions.
The attraction of this concept is that it may offer a way to build four-stroke engines with the efficiency of a diesel, and without an NOx/soot problem.
If an HCCI engine were designed \ to run at a single load level-as engines do in some hybrid combustion/electric vehicles-load control would become a non-issue because all we need to do is recirculate enough hot exhaust gas to make the engine ignite at the desired point in its cycle. But if we want engines to idle, accelerate and then cruise at any desired throttle opening, we have to find rapid, cost-effective means of varying the auto-ignition point of the mixture as it is compressed, over the full range of load. Currently being discussed are hot exhaust gas recirculation, variable valve timing, active radical injection and variable compression ratio.
As you may imagine, substantial rewards await the organization that is able to deliver the diesel’s low fuel consumption combined with the relative freedom from soot and NOx enjoyed by today’s most highly developed gasoline engines. Detroit enjoys its highest per-vehicle profit from large, heavy SUVs, but their high fuel consumption and other issues are generating strong opposition. A new engine type, with the torque and fuel economy of a diesel, but able to meet all applicable emissions standards, would give this vehicle new viability.
Think farther ahead. Will there be hydrogen fuel cell motorcycles soon? Because hydrogen storage is bulky, not likely, even if hydrogen somehow becomes cheaply available. A better internal combustion engine? This is the likely outcome, because combustion engines can use the present systems of fuel refining and distribution. Present-day gasoline engines are good, but they will improve. HCCI is one possible future direction.
