LA Metro Buys 96 More Natural Gas Buses
Real World: the E85 Hunt

Proposal: Bringing HCCI to Market with an Overexpanded Two-Stroke Cycle Engine

A sketch of the overexpanded two-stroke engine concept. The compressor (see below) is element #25.

Engines utilizing homogeneous charge compression ignition (HCCI) combustion regimes offer the promise of being cleaner-burning and more fuel-efficient than current engines—and hence are attracting a great deal of research attention and funding, especially for engines destined for heavy-duty transportation applications. (Earlier post.)

HCCI is based on the concept of the autoignition of an entire compressed fuel/air mixture—but controlling the timing of autoignition for complete burn is difficult. One researcher is proposing a short-cut for bringing HCCI engines from the laboratory to the marketplace, however: the use of an overexpanded two-stroke HCCI cycle engine.

Dr. Pao Chi Pien presented his notion of an overexpanded two-stroke cycle as a platform for Otto, Diesel and HCCI combustion modes at SAE’s Future Transportation Technology Conference last September.

The basic overexpanded two-stroke cycle consists of:

  • A constant-volume, constant-pressure or constant-temperature combustion process (or a combination of them)

  • An overexpanded expansion process (e.g., like the Miller or Atkinson cycle)

  • A shortened exhaust process of a four-stroke engine cycle, which is compensated for by a scavenging process of a two-stroke engine cycle.

The proposed HCCI application of this basic design utilizes a three-stage fuel injection process, with a two-part compression process.

  • Stage 1: Fuel is injected under low-pressure into a separate air compressor to provide a partially compressed lean homogeneous charge upon intake to the engine cylinder. The upward movement of the piston further compresses the charge to reach a temperature just below the autoignition threshold.

  • Stage 2: A small amount of fuel is injected just prior to the piston reaching top dead center (TDC)) to trigger autoignition when the piston reaches TDC.

  • Stage 3: Following autoignition of the lean homogeneous charge, a third injection is made to achieve additional combustion at a constant temperature to generate additional power without producing NOx.

Pien asserts that the key to controlling the autoignition is the predictability of the in-cylinder temperature of the charge made possible by the characteristics of the overexpanded two-stroke engine.

The overexpansion compensates for what would otherwise be a reduction in thermal efficiency resulting from the combustion process. According to Pien’s calculations, for the same displacement as a four-stroke engine, the two-stroke HCCI engine could deliver a fuel savings of 36%.

For [an] automotive truck application, an overexpanded two-stroke HCCI engine can be provide enough power to maintain a speed of 70 mph on the highway.

—Pao Chi Pien



Rafael Seidl

Two-stroke crosshead turbodiesels are standard on big ships where you want to avoid a transmission and connect the propeller directly to the driveshaft. Rotation speeds are ~100-150 RPM, with power ratings of up to 100,000 bhp. Efficiencies in steady-state can reach 52%.

The proposal here is for a much smaller two-stroke diesel running at much higher RPM *and* using volume ignition. Like a four-stroke, it features a full valvetrain (i.e. same or higher cost of manufacturing for the engine as a whole). Head scavenging is known to operate poorly, yielding high internal AGR rates.

The pilot injection is performed immediately after closing the intake valve, cooling the gas as it evaporates the fuel. The objective is a homogenous pre-mix by the time the piston gets close to TDC. A second injection event produces clouds of locally enriched air-fuel mixture, providing a large number of ignition foci - just what you want for HCCI. Btw, high-pressure common-rail systems with piezo injectors can deliver multiple events.

The first problem is that you inevitably suffer large cycle-to-cycle variations in your AGR rate and hence, in the temperature the pre-mix reaches just before the second injection. Execute the second injection too early and the cloud disperses so much that voluem ignition never happens. Execute it too late and you trigger regular compression ignition with a flame front. It may be possible to manage this balancing act in steady-state but transients are much more difficult.

The second problem arises from the high heat release rate associated with volume ignition. While isochoric combustion is thermodynamically advantageous, the associated pressure pulse places extreme stresses on the crank-slider mechanism, even as the surfaces are exposed to extreme radiative heat transfer. This combination, along with the high noise level, limits HCCI concepts to low partial load scenarios. Note that high internal AGR rates imply the walls, piston crown and poppet valves need to be cooled very aggressively relative to the delivered power, aggravating quenching effects. This leads to elevated HC, CO and PM emissions.

The author then suggests a third injection to burn off these products of incomplete combustion. However, it is quite unclear how you would prevent a flame front from forming given the high temperature and pressure of the gas immediately following volume ignition of the fuel injected previously. Hence, I would expect this third injection to produce some small amount of NOx.

Finally, the gas is allowed to overexpand to a volume greater than it had when the intake valves last closed. In the context of a two-stroke, this implies an asymmetric control diagram, i.e the valve event order is exhaust open, intake open, exhaust close, intake close. The sequence is started ~55 deg crankshaft before BDC and end ~85 deg CS after BDC.

This is easily implented with a full valvetrain. However, the temperature of the exhaust gases will be quite low. The turbocharger needs to be extremely efficient to achieve adequate boost pressures. This is the case for those marine diesels which operate at constant high load most of the time. It is almost impossibe for small turbines designed to support the high load dynamics of a vehicle engine. Ergo, you'd need to at least supplement with a mechanical or electric supercharger, adding yet more cost.

Final note: AVL, a multinational engineering consultancy, actually designed a small three-cylinder two-stroke turbodiesel in the 90s, using regular compression ignition. The benefits were deemed too slight to warrant series production. AVL's preferred volume ignition strategy - tested on four-stroke engines - is called HPLI and roughly comparable to the one described here. Combining two marginal ideas does not neccessarily yield a great one.

tom deplume

I like the over expansion concept whether it uses HCCI or not. Supercharged uniflow diesels have been used in trucks and buses since the 1930s. These used an intake port at the bottom of the cylinder and 4 exhaust valves in the head. That it still hasn't been used in a spark ignition direct injection escapes me.

richard schumacher

Can one replace the spark plug in a conventional engine with an antenna (making the cylinder a microwave cavity), and ignite the fuel/air mixture uniformly throughout with microwaves?

richard schumacher

Nahh: the power required of the microwave source to heat the mixture fast enough would be unreasonable, much more than 10 kW.


I still say a two-stroke, two cylinder, turbocharged, valveless, diesel running at it's most efficient designed speed - connected to a generator - is the only way to go.

The technology is there. All it would take would be a little design experimention.

joe padula

All the makers of what you have described have stopped making them. GM's Detroit diesel 53,71,149 series for boats trucks and busses. GM's EMD locomotive and tugboat sized 567, 645, and 710 series.
Sulzer stopped valveless engines with the RND series in the early 80's.
No one can meet the pollution requirements with these engines.
The only 2 cycles left are the lawn care, outboard boat and model airplane engines.
In ships 2 cycles are still used because there is basicly NO pollution requirements other than a joke level of NOX EPA catagory 3.

Thank you for the great explaination!

Bud Johns

Joe, the emd's and all of GM'S diesels (such as the 71 series) had intake ports that the pistons uncovered, and four exhaust valves at the top. They must have a blower to run. The first ones had roots blowers, the later more powerful and efficent ones had turbos that had to be gear driven until the engine speed was sufficient to provide enough heat to the turbo. In medium size diesels like this, it has been found that four strokes are more efficient, and GM has switched over. As for the giant ship engines, two strokes are the way to go and are the most efficient prime movers in the world today.


Here is another small sized two stroke diesel concept: the Z engine. It remains to be seen whether it can fulfill its promises.

Rafael Seidl

jb -

the Z engine implements 4 cycles in a single crankshaft rotation. It is able to do so because it relies on external compression using mechanical superchargers (make that super-duper-chargers). The peak combustion temperature is reduced via agressive intercooling of the fresh charge - which is a good idea.

Unfortunately, mechanical superchargers tend to have a negative impact on fuel economy. In a multi-stage compressor, as would be sensible here, one or more stages can of course be turbochargers, to mitigate this effect. Cp. BMW's new 535d or Borg-Warner's e-Booster concept. Turbos used in truck engines can deliver 4x the inlet pressure in a narrow operating range.

Note, however, that the exhaust valve is closed half-way between BDC and TDC, i.e. the chamber is filled with combustion gases with nowhere to go. Hence, internal AGR rates are inhrently high. This reduces NOx production at the expense of PM formation and fuel economy.

A third problem is that the time available for introducing the fresh charge into the cylinder is very short, which sharply limits the maximum RPM. Using supersonic flows past an inlet valve as suggested would create shock waves inside the cylinder, increasing mechanical stress, noise and heating the fresh charge - none of these are desireable.

Moreover, the surfaces of the combustion chamber are only exposed to relatively cold, fresh charge for a small fraction of the time. This leads to high cooling requirements, especially for the piston crown and the exhaust valves. Given that the valves are actuated twice as often as in a regular four-stroke engine, their life expectancy probably represents a difficult problem.

Conclusion: I doubt this design would deliver greater specific power than a regular four-stroke yet cost more.



Can one replace the spark plug in a conventional engine with an antenna (making the cylinder a microwave cavity), and ignite the fuel/air mixture uniformly throughout with microwaves?

I have seen some designs where the spark is replaced with a laser, or more specifically a ring of lasers. This allows more uniform combustion and almost zero's out NOx emissions. As with most advanced technologies, it's only in development and who he heck knows if it will ever come to fruition ;-)



Joe - actually, the EMD 710 engine is still in production (used in their SD70 locomotives) and is Tier 2 certified (

Rafael - I'd also like to thank you for your very informative posts.

tom deplume

Radar uses microsecond long pulses of several megawatts while the average power draw of the system maybe only a few kilowatts. The problem with microwave ignition is the need for a magnetron or power klystron for each cylinder. Spark plugs are extremely cheap compare to the cost of magnetrons. There are easier ways to improve engine efficiency and cleaning the exhaust than HCCI.

The comments to this entry are closed.