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.