The trouble with a spark assisted low compression "diesel" is; it isn't really
a true diesel.

True diesels ignite from high heat and the high heat comes from the high
compression ratio as does the high efficiency and low heat rejection
to the coolant. A high mechanical compression ratio wankel rotary would have
a poorly shaped combustion chamber. Now if it was external compression
(a turbo charger) that could be another matter but unfortunately the high heat is
wasted in the intercooler. Now if we could only control the detonation
of a turbo rotary with out intercooler than maybe.....

Turbo compound is the simple answer.

Paul Lamar

The Diesel cycle is not constrained to compression ignition, but relates to a constant pressure heat
addition during expansion. So called Diesels using compression ignition, have less heat rejection
because they burn less fuel per charge, for a lower total heat input. A normally aspirated "Diesel"
engine will have less torque ( and less HP) at any given rpm than the same size gasoline (Otto
cycle) engine. Better efficiency is obtained because they are able to run a higher compression
(really expansion) ratio than a spark ignited engine. Also at light load, they run way leaner
than any spark ignited engine, and have less pumping loss. For the same compression (expansion)
ratio, the Otto cycle has a higher thermal efficiency than the Diesel cycle. In turn the Brayton
cycle
(gas turbine) has an even higher thermal efficiency. In the real world, they have to run where they
can
live, and the results are what you see. A direct injection stratified charge gasoline engine has many
of the same advantages and disadvantages as a "Diesel".
Vance

"So called Diesels using compression ignition"?????

Why do you say "So called" Vance? The vast majority of diesels (99.9%) we are familiar
with have compression ignition and compression ratios of 15:1 or higher.

Paul Lamar

The reason I say "so called" is that modern diesels do not operate within the guidelines of the Diesel
cycle. Rudolf envisioned an engine which operated by adding the heat to the working fluid at a
constant pressure as the volume increased during the power stroke. The Otto cycle (attributed
to spark ignited engines) assumes heat addition at a constant volume (the short period of time
at top dead center). In the real world, neither of these cycles is followed for practical reasons,
and they both operate in a bastardized combination of the two cycles. The spark ignited engine
obviously lights the fire before tdc, but the combustion process is still continuing beyond tdc.
In a very like manner, the compression ignition engine injects fuel early, and much of the fuel is
combusted before expansion starts, but undoubtedly continues longer than the premixed spark
ignited engine case, particularly at high power settings where the injection process extends for
more degrees of time.A direct injection, stratified charge engine operates just as close to a true
Diesel cycle as a compression ignition engine. The Otto cycle is a higher thermal efficiency cycle
than The Diesel cycle, but in practice the higher possible expansion ratio of the compression ignition
engine, and the reduced pumping loss give the "Diesels" the advantage in most operating regimes.
Is a model glow plug engine a Diesel ? is the model "Diesel" engine - with a premixed charge
a Diesel? If you could convince an engine to operate as Herr Diesel envisioned, it would have lower peak
pressure for an equivalent BMEP and would require less robust structure.
Vance

 
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