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Concept: Sleeve Axial Diesel Engine
29 January 2007
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| View of the Shepherd concept engine. Click to enlarge. |
A New Zealander is developing a new concept sleeve axial diesel engine designed to increase engine efficiency, deliver a high power to weight ratio and deliver lower fuel consumption than conventional diesels.
The “Shepherd Engine,” created by Graydon Shepherd, features fixed pistons and reciprocating cylinders—conceptually the inverse of a free-piston engine in which combustion at alternating cylinder ends drives a piston assembly back and forth. (Earlier post.) The Shepherd engine converts the reciprocating motion to rotary without the use of a crankshaft.
A proof of concept engine has been demonstrable since 2005, and Shepherd is endeavoring to develop the concept further, now supported by Unitec NZ. A third prototype is due to be built early this year.
The main support sleeve of the concept engine supports two fixed pistons, one at each end. Diesel is injected through injectors in the piston, and the combustion drives the cylinder sleeve backward and forwards. The motion of the sleeve is controlled by slides running in axial slots in the stationary engine outer housing.
A pair of cam followers drives a cam profile on each end of the rotating outer sleeve, converting the axial reciprocating motion of the reciprocating sleeve into the rotary output motion of the rotating outer sleeve. The rotating outer sleeve also acts as a flywheel. Two reciprocations of the cylinder sleeve are required to complete one rotation of the rotating sleeve.
Each end of the reciprocating sleeve includes an approximately spherical pre-combustion chamber into which diesel fuel is injected from the rear of the fixed piston just before or at the beginning of the power stroke. During the compression stroke the motion of the reciprocating sleeve scavenges air into the pre-charge chamber through holes in the fixed outer housing.
The motion of the reciprocating sleeve slightly pre-compresses the combustion air in the pre-charge chamber; the charge then is transferred to the combustion chamber via a transfer valve. According to an analysis of the engine by Professor Stephen Johnston of the Applied Technology Institute, Unitec NZ, this is a key feature.
The engine layout allows the diameter of the pre-charge chamber to be larger than that of the combustion chamber, allowing a degree of pre-compression (chosen during design). This pre-compression allows for improved purging of the products of combustion from the combustion chamber and/or the possibility of a degree of supercharging. In combination with pre-compression, a system of porting means that the engine design eliminates the need for valves and for a drive train to operate them. The almost doubled volume of scavenged air also allows for a measure of internal cooling from within the combustion cylinder itself and it is also expected that the use of ceramic components in conjunction with steam injection will aid in the reduction of nitrous oxide and particulate matter emissions as development proceeds.
—Stephen Johnston
In a conventional engine the majority of the power is extracted from the cylinder while the throw of the crankshaft is between two- and four-o'clock. The shape of the output torque curve in the Shepherd engine is not limited by this constraint, and the engine is able to extract power efficiently form the reciprocating sleeve over a greater portion of the movement, thus producing more uniform torque, according to Johnston.
The combustion force is also transmitted to the flywheel via a greater moment arm (or leverage) than in a conventional engine design. In a conventional engine of similar power the combustion force is transmitted from the piston, down through the connecting rod to the crankshaft with a maximum moment arm (crank throw) on the crankshaft of about 75 mm. The transmission of combustion force on a comparable Shepherd engine is at a constant moment arm of 150 mm, providing much greater torque. Eliminating the sideways forces associated with crank-driven engines minimizes friction between the pistons and cylinder walls, minimizing cylinder lubrication requirements, reducing oil requirements, and increasing energy efficiency.
—Stephen Johnston
Incrementally moving the position of the fixed pistons relative to the reciprocating sleeve will change the compression ratio, opening up the possibility of operation on a range of fuels.
January 29, 2007 in Concept Engines | Permalink | Comments (3) | TrackBack (0)
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Comments
Cutaway diagrams of this concept at various stages of the cycle would be helpful. A picture is worth a thousand words, and the photo is not particularly helpful. Note that such undulating cams do of course produce forces normal to the centerline of the cylinder, loading the piston rings. In this regard, the design offers no advantages over a traditional crank-slider mechanism, precisely because the reciprocating element is in fact not free but constrained by the cam.
In general, I also see no advantage to using a reciprocating cylinder instead of reciprocating pistons. The weight is greater, lubricant sealing is harder and the space available for the intake and exhaust manifolds is constrained. It's unclear how the exhaust manifold is cooled to achieve adequate life expectancy and prevent coking. Horizontal setups are particularly tricky for two-strokes for this reason.
Posted by: Rafael Seidl | Jan 29, 2007 1:59:44 PM
I think the useful life of the cam follower is not good enough.
To me, the conventional connecting rod - crankshaft mechanism is much more reliable and durable.
If a big diesel truck is driven carefully and given proper preventative maintenance, the engine can last for more than 1 million Km. (621,000 miles) without any major repair job.
Posted by: Jorge | Jan 29, 2007 3:37:24 PM
Read the "Resources".
This is still at a very early stage. Should it prove to be an advance - as they indicate - in the ICE, more power to them. I would like to see it connected to a generator with constant speed controls.
Might make a highly efficient "genset".
Posted by: Lucas | Jan 29, 2007 3:44:19 PM
