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Grail Engine Adopts Enerpulse Pulse Plugs for Forced Semi-Homogeneous Charged Compression Ignition in Concept Two-Stroke Engine

Rendering of the Grail Engine. Click to enlarge.

Grail Engine Technologies, the designer of a two-stroke engine using forced semi-homogeneous charged compression ignition (FS-HCCI) combustion, has adopted the Pulstar pulse plugs from Enerpulse (earlier post). The Pulstar product offers very high power spark discharge, on the order of 1MW, to accelerate combustion pressures enabling forced semi-homogeneous combustion for all conditions.

HCCI is a combustion regime in which well-mixed fuel, exhaust gas and air are compressed to the point of auto-ignition. Unlike a spark ignition gas engine or diesel engine, HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously. HCCI combustion can deliver a very efficient engine, potentially providing a 20% to 30% boost in gasoline engine efficiency without the NOx or PM emissions of a diesel.

Hurdles facing HCCI implementation include the difficulty of control, a limited power range and incomplete combustion; the Grail Engine is being designed to overcome these hurdles, according to Matthew Riley, CEO and Chief Research Scientist at Grail Engine Technologies.

By incorporating multiple, high-energy, rapid discharge points in the cylinder, HCCI can be forced allowing combustion to be controlled. This is the key to setting a new standard of fuel efficiency and emissions reductions. A 1-liter 2-cylinder Grail Engine using Pulstar pulse plugs is expected to yield 100+ mpg without batteries or power grid [Note: Using a commuter vehicle weighing 1,600 lbs/726 kg]. In different configuration the same displacement 1-liter engine can yield 200 hp [149 kW] and 180 lb-ft [244 N·m] of torque. The first 2-cylinder advanced prototype of the Grail Engine will be available for demonstration and testing in the 4th quarter 2010.

—Matthew Riley

Elements of the Grail Engine. Click to enlarge.

According to a description of the Grail Pneumatic Two-Stroke Engine for the NASA Create the Future Design Contest 2008, the proposed two-stroke engine is based on the use of a piston assembly which includes the piston, a piston check valve, and piston intake vents.

Intake occurs when the piston moves up by creating a vacuum within the engine crankcase beneath the piston and piston check valve. A one-way intake reed valve & throttle plate (IRVTP) opens to allow outside ambient air to enter the crankcase. As the piston approaches Top Dead Center (TDC), the direct fuel injection system injects the fuel charge. Ignition occurs via the sparkplugs. Expansion forces the piston down, compressing the air in the crankcase below. Just before Bottom Dead Center (BDC), the exhaust valve opens, via a standard cam valve train (once per revolution of crankshaft.)

Compressed air in the crankcase beneath the piston travels through piston intake vents, the piston, and passes the piston check valve into the combustion chamber, forcing final exhaust and fresh air into the exhaust port. Just past BDC, the piston check valve and exhaust valve closes and cycles repeat.

The piston check valve operates on light spring and pneumatic air pressure between the combustion chamber and the crankcase. The piston intake vents extend like straws toward the center of the crank shaft at (BDC). Air is forced through the piston intake vents by the pressure in the crankcase. Oil is kept away from center of crank shaft by inertia and centrifugal force.

Detonation force (pressure) maintains the seal between the piston check valve and the piston. The piston check valve covers more than two-thirds of the piston top surface area. In his original design, Riley suggested that the check valve can be made of thin light titanium to help minimize inertia overthrow for higher RPM engines.

The piston check valve and the piston are cooled by incoming compressed air, also achieving Positive Crankcase Ventilation (PCV). Exhaust valve management allows the engine to independently configure (Intake Compression) to (Power Exhaust) ratios. This design allows for more than four ignition points in series or parallel depending on power or semi-homogeneous ignition when desired, according to Riley.



Will this be one more wonder ICE? Will it follow the same path as the Scuderi Cycle and many other wonderworks?

Would the world change that much if wonderworks ICE, with higher efficiency (about 38% instead of 25%), were widely used?

Massive Vehicle electrification, with 80% + efficiency, may be a much better solution.

Henry Gibson

Oh come now, Scuderi now has a single piston set prototype under test after ten years.

There are enough reports on the internet to show that simple hydraulic hybrids as well as electric hybrids can cut fuel use by as much as %50 without even using smaller more efficient engines. Small increases in fuel efficiency are not a demand of buyers. Diesel engines can get high power and high efficiency. The OPOC can deliver diesel efficiency at high power and use exhaust filters etc. Or a Tiny version of the 80 year old JUMO aircraft engine could be built.

If there is not enough engineering talent at all of the engine builders to build small single piston Diesel range extending generators, then the automotive world does need help. But the real problem is that there is no law that people must use efficient vehicles like they must use efficient shower heads. At least the second and third automobiles in a family should be small and efficient. A Hummer is not needed to drive to the office, and neither is 0 to 60 in six seconds.

Yes, electric vehicles are practical, but various interests, including a demand for very high power, keep them off the road because of the high expense of their introduction in small numbers by many companies at very high prices, and all the publicity that increases range anxiety when tiny range extenders would eliminate it for most users.

Henry Gibson

Perhaps a new precombustion chamber can be engineered to give a high power pulse. It could use on board hydrogen generators to use the known explosive effects of hydrogen and please the hydrogen folks. Some of the first engines opened a port to a gas flame for ignition, similar to an old cannon. ..HG..


I'm with HarveyD.
Except if there really were an engine that got 13% better efficiency, it would be a big deal - whether we like it or not.


While I agree that it is extremely difficult for a new concept of ICE to make it through I aslo reckon that there is a need for compact light very efficient and clean engine for range extender. That's where new concept can emerge. For standard application, existing technologies can still be improved with direct injection, turbo-charging, down-sizing, reduced number of cylinder, Atkison. For those who think that electric will be the main stream 10 years from now, have nice dreams, Santa Clauz will take care of your fancy wishes...

Fred H

Conventional cars require engines that deliver smooth, responsive power over a power range that spans two orders of magnitude for good driveability. The design of the Grail engine, and many other engines, prohibits this to be realised without sacrificing manufacturing economy, and/or emissions, and/or efficiency.

If they try to make it work cleanly, efficiently, and smoothly over the power range necessary for an acceptable driveability in a conventional car, they will end up where the “clean diesels” are now: Multiple variable vane turbochargers and diversion valves, and intercoolers, and variable valve lift and timing, and exhaust gas recirculation, and ultra high pressure direct injection, and throttles, and waste gates, and multiple catalytic converters, and exhaust filters, and injection systems into the exhaust, and filter burn out cycles, etc. All this, and more, is necessary to smooth out the torque curve, shorten the response time, reduce the emissions, and increase the efficiency during the times when the engine is not able to operate in its inherently small optimum range.

In a hybrid car, the engine could run in its inherently optimum range more of the time. Unfortunately, Matthew Riley said
“A 1-liter 2-cylinder Grail Engine using Pulstar pulse plugs is expected to yield 100+ mpg without batteries or power grid [Note: Using a commuter vehicle weighing 1,600 lbs/726 kg].”
which I interpret to imply that they want to try to make it driveable in a non-hybrid. (note: with low emissions and driveability as a prerequisite, I suspect that mileage figure could be achieved only at steady speed on a level road – not your typical driving)

Dear Grail Engine Technologies, please do not waste too much time trying to make your engine adequate for a non-hybrid car. When it comes to real world driving, you can’t beat a hybrid car that lets its combustion engine run in the optimum range. If you can’t beat ‘em, join ‘em. Team up with a hybrid drive train systems developer, and configure a system that gets the most out of your engine at its inherent optimum range.


Good points Frd but I think that constant speed is often overrated as an efficiency driver.

Braking and re-acceleration drive city mileage down.

Wind resistance takes its toll on the highway.

Most any car gets surprising mpg at a constant low speed without tailoring the engine.

Even more so as such things as VVT become affordable.

Vincent Armstrong

Think about this for just a minute....... IF this works as well as it is billed the technology can be used in a much larger engine. I for one would love to see this running and test it personally. In the "Grail" one cyl creating 170-180 hp(hp numbers from Grail Engine Technologies) there are endless uses. Add two or three more cyl and you are looking at 340-510 hp. To me a three cyl @ 340hp........ WOW. I live in Kansas and 45min north of the creators. Let me test it and see what the longevity of this engine is as I drive about 48,000 miles a year for work.

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