A unique engine, based on the regenerative principle, is being
developed with the goal of achieving high brake efficiency over a
wide power range. It can be characterized as an internal combustion
Stirling engine (ICSE). The engine is a split-cycle configuration
with a regenerator between the intake/compression cylinder and the
power/exhaust cylinder. The regenerator acts as a counter-flow heat
exchanger. During exhaust, the hot gases are cooled by the
regenerator. The regenerator stores this heat. On the next cycle,
compressed gases flow in the opposite direction and are heated by
the regenerator. The gases coming from the regenerator into the
power cylinder are very hot (~900°C), which provides the necessary
gas temperature for auto-ignition of diesel and other fuels.
A simplified Air Cycle analysis of the ICS engine is presented
to validate the concept thermodynamics and to show the inherent
difference between the ICS and conventional internal combustion
engine (ICE) indicated efficiency. The ICE engine indicated
efficiency increases with increasing compression ratio and is
insensitive to peak temperatures, whereas in the ICS engine
indicated efficiency increases with decreasing compression ratio
and increasing peak temperature. This engine concept is a candidate
for application of adiabatic engine technology which has been
explored for many years. With materials that can withstand high
temperatures, brake efficiencies of 60-70% are possible. Low heat
transfer is important to the proper operation of the engine.
A multi-step cycle computer indicated thermodynamic and fluid
flow model of the ICS engine of increasing detail was used during
the engine development. Finally, detailed perturbation studies were
conducted to fully understand the ICS design sensitivities. An
engine friction model was added to the computer model to be able to
compare estimates of ICSE BSFC and BMEP with ICE engines.
Important ICS engine innovations include elimination of
throttling losses, low friction due to low compression ratio, and
very high air cycle efficiencies (~80%) combined with low
compression ratio. The engine is designed for the highest possible
efficiencies. In addition to these advantages, the engine has
nearly constant pressure combustion, which should help reduce NOx
formation.
The major findings were: the ICS engine is more efficient than
either gasoline or diesel engines over the entire operating range
especially at part power. At wide open throttle, an ICS engine is
more efficient than either a gasoline or a diesel engine. This
advantage increases at part power. On the negative side, the ICS
engine has inherent low power density (volumetric efficiency)
because of low compression ratio, late air intake and late
combustion. A prototype engine and a modest engine test dynamometer
and instrumentation are nearing completion to demonstrate the
P&B Enterprises, Inc. (PBEI), ICSE concept. The prototype is a
retrofitted two-cylinder diesel engine. The prototype uses the
existing engine block, and the crankshaft and camshaft fit into
existing spaces in the block. Anticipated problems to be addressed
with the prototype engine are starting, combustion characteristics,
regenerator temperature control and high turbocharging ratios to
achieve reasonable power density.