Single-cylinder, two-stroke engines are some of the most common and compact powerplants in the world. Powering everything from weed-whackers to Supercross motorcycles these simple engines offer impressive power delivery for tiny displacement.

We discovered a unique approach to modernizing a system that has remained nearly unchanged for over a century. The folks at Grail Engines pioneered a prototype that uses clever thinking, packaging, and design to achieve power and efficiency unheard-of in two-strokes.

Photo Credit: Grail Engine Technologies

We reached out to the Kansas-based firm to find out more on these unique little engines, only to find that unfortunately the operation has dissolved. We are however left with some thought provoking animations, and interviews from the creators.

Observing the dynamic animation of the Grail engine we see that it operates like a traditional two-stroke with several unique features. Both reed, and traditional stemmed valves control the directional flow of air, fuel and exhaust gasses as the engine runs.

The initial intake of air is drawn past a reed valve and down a vertical runner into the crank case. Unlike a traditional two-stroke which uses a sleeve valve arrangement to uncover and cover ports, the air/fuel charge is swept into the cavity between the 360 degree crankshaft counterweights. By the time the piston reaches BDC the full charge is contained (compressed) underneath the piston.

As the piston begins the compression stroke, a valve in the face of the piston releases the high pressure A/F mix into the low pressure cylinder where it begins to be compressed again — simultaneously a pushrod actuated exhaust valve in the cylinder head allows the burnt gases to escape. The power stroke is shared by the intake stroke but the two events are separated by the partition of the piston.

The potential of this design is compounded when considering banks of cylinders joined together, and the advertised “green” approach to emissions regulations. “The Grail engine has the potential to be the first two-stroke engine that does not exhibit cross-contamination of fuel and oil. This results in lower emissions yet produces more power and torque using less fuel than larger engines,” according to the Grail website.

“The engine is a 500 cc, but since it hits every revolution it will produce what a normal 1-liter engine will output. It’s capabilities are approximately 100 miles per gallon, and 100 horsepower,” outlined Mathew Riley CEO of Grail Engines.

Photo Credit: Grail Engine Technologies

Such efficiencies are closely tied to the ignition system employed by the Grail design — working with Pulstar for plugs they are able to use a method called forced semi-homogeneous ignition technology. By utilizing multiple ignition sources in the compression chamber, and employing Pulstar’s patented technology for plasma generating ignition plugs Grail is able to make impressive claims.

“Homogeneous or HCCI (homogeneous charged compression ignition) means everything ignites at once, forced semi-homogeneous means that the ignition bloom is increasing rapidly enough to raise the compression in the chamber to detonate the remaining air and fuel simultaneously,” explained Riley.

While the Grail Engine may not have taken off, it is an example of some very creative design and engineering. If not for automotive use maybe we can find applications for this sort of powerplant in industrial or motorcycle use.

Photo Credit: Grail Engine Technologies