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My bright idea.... reader Glen Johnston, a retired engineer, talks 'turblown' engine solutions.

The key to completing the most efficient fuel combustion reactions is 'the three Ts': more time, more turbulence and more temperature. In reciprocating engines, the time available for combustion is non-negotiable as a design variable, but much has been achieved by increasing turbulence to probably its design limit.

However, maintaining combustion temperature throughout the operating envelope has been problematic other than for compression-ignition diesels. Petrol-fuelled engines require fixed fuel and air ratios. Thus, at part load, the total volume of fuel/air mixture allowed to be drawn into the cylinders must be controlled. So the effective compression ratio, and therefore pre-combustion temperature, are reduced at every stage from full load down to idle. Simplistically, at 50% load, an engine designed for a compression ration of 10:1 will in fact be operating at 5:1.

The answer is a constant 10:1 compression ratio at all loads, achieved by mechanically separating the combustion space from the rest of the air compressed in the cylinder. The ascending piston compresses the air charge into two smaller cylinders, each 10% of the swept volume via ports. The smaller cylinders themselves each contain pistons that are hydraulically, or mechanically, linked. One small cylinder is the combustion space; the other is the excess air. Metered fuel is directly injected at low pressure at the beginning of compression into the combustion cylinder, whose piston position is linked to the quantity of fuel being injected to give the correct fuel-air ratio. As the power piston ascends, it forces air into both cylinders in the correct proportions until it is at top dead centre, sealing off the ports to each.

Conventional spark ignition in the combustion cylinder then expands through the port into the power cylinder. The hydraulic link between the two small cylinders is effected by another positioning piston separating the ports to each. However, the net force experienced by the positioning piston is zero, since the same combustion pressure exists on each side but in opposite directions. It's easier in two-stroke form as the cylinder head has to accommodate the smaller cylinders so there is little space for valves, and it's 'turblown' as it doesn't use the crank-case for air charge transfer, thus avoiding oil carryover and contamination.

But why not turbo-charged? Because you can't turbocharge a fixed port two-stroke, as both inlet and exhaust ports are open at the same time and the inlet closes first. So you can only purge the combusted cylinder of exhaust products and refill it with fresh air.


The constant compression ratio achieved at all loads

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Title Annotation:Engineering extras: Innovations: Ideas: Careers: Gadgets: Books: Internet
Publication:Professional Engineering Magazine
Date:Jun 1, 2015
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