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As we all know, the key determinant in any internal
combustion engine's efficiency is its expansion ratio. This
because this ratio denotes how much the mixture of fuel
air is allowed to expand after combustion, and thus how
much energy is actually converted to work vs lost to heat.
common misconception is that the thermal efficiency of
engine is determined by its compression ratio, which
sense since the mechanical, static compression ratio is
to the expansion ratio. However, the key thing to note is
that while expansion ratios increasing beyond, say 15:1,
continue to yield increases in thermal efficiency,
the compression ratio past that point leads to much higher
frictional or "pumping" losses.
The diesel engine is known as achieving around 40%
efficiency vs. 25% for the otto cycle. Besides perhaps a few
percentage points that are accounted for by the fact that
diesel engines are not throttled and lose no energy to
vacuum, this increase comes purely from the increase in
expansion ratio. And what's more, this increase in
ratio is not even overly profound (avg diesel
compression/expansion ratio is ~16.5:1-18:1 vs up to
for direct injection gasoline). Part of the reason diesel
engines run out of steam so early in the RPM range is
because the pumping losses from compressing the mixture
increase drastically as the revs climb.
The Atkinson Cycle engine works by physically closing the
intake valves late to allow a certain volume of air to flow
back out. This lowers the "dynamic" compression ratio, and
increases thermal efficiency because the expansion stroke
larger than the compression, but at the cost of reduced
power density. If the engine is a 3L, and we end up with a
maximum of 8/10 original volume left after the Atkinson
cycle, you only effectively have a 2.4L.
I propose a highly exaggerated Atkinson Cycle; one that
would have an abysmal effective volume and
so great power density, but extremely high thermal
efficiency (~70% or greater). What's more, there's really
nothing proprietary here; this could be done easily on
existing internal combustion engines. It just sounds like a
really bad idea until explained.
In my highly exaggerated Atkinson Cycle Engine, the static
compression ratio (thus expansion) is 75:1. However, the
Atkinson Cycle is implemented in such a way that the
can only ever hold 2/10 of its original volume; thus a
dynamic compression ratio of 15:1 maximum at full load.
the engine, there would be cam phasing such that up to
95% of air sucked in is expelled back out during
compression. All of this means that even at maximum
load, the engine is working extremely little at
compressing the charge compared to what it harnesses as
On the surface, the engine would appear to have
characteristics of extremely poor power density. 2/10
original volume means a 2L engine will now only hold
400ccs. However, because of the stratospheric expansion
ratio, I predict thermal efficiency in the ~70% range. So
while the power density on the surface is only 2/10
because the efficiency increases 3-fold compared to an
Otto Cycle, the power would also. This would mean
around 60% power density vs. an average engine of the
same size (which is already character, and most
3x the thermal efficiency.
Not quite exaggerated
[MaxwellBuchanan, Mar 26 2012]
Some good background info
[scad mientist, Mar 27 2012]
||Excellent thermal efficiency, but lousy weight and size package.
||Eh. Lousy by what standards? Diesels already only
have 60% power density vs. Otto cycles because of
their inability to run anywhere close to
stoichiometric (with any semblance of clean
burning; without massive soot clouds).
||So, basically, my idea would have identical power
density, but would not need the needlessly heavy
internals that a diesel needs to handle compression
||Also, it is my opinion that in real world situations,
this difference in weight would be negligible unless
it required a greater cylinder count. Let's take a
Honda Fit; powered by a 1.5L engine, and multiply
that engine's displacement by 10/6 to find required
displacement of this idea.
||You get 2.5L; which nearly matches the Honda K24
engine (2.4L). I am having trouble finding exact
specs, but I would have a very difficult time
believing the difference in weight between the two
aluminum block 4-cylinders would be anything
greater than, say, 40-50lbs.
||I thought that this was an idea for being forced to eat large amounts of meat whilst on an exercise bike.
Until I discovered Smirnoff
||The link I added to Wikipedia shows that this these concepts have been researched, but stuff I read online seems to be heavily biased with the assumption that reducing power output is bad. You seem to be looking at it from a better perspective that upgrading the engine size to maintain adequate power may be worthwhile for the efficiency gains.
||I'm looking into buying a new car for commuting. What I'd really like is a small two seat "sports" car, but with the engine of an economy car. Most car companies in the USA sell economy cars and sports cars. The sports cars generally have smaller dimensions but larger engines. I'd like to get the small engine in the small car, resulting in better acceleration and fuel economy than the economy car, not to mention the nicer looks.
||This idea gets me thinking: Would it be possible to use the standard large engine in the sports car and replace the cam to make it more efficient than the economy car engine? I've heard that you can buy racing cams to boost your horespower. Has anyone heard of any company selling an econo-cam? I assume there might be lots of other modification needed to make this work well. Boosting horsepower can be done rather crudely if you don't care too much about efficiency, emmisions, engine life, etc. I would want to trade off power for fuel economy, but maintain (or improve) emmisions and engine life.
||It's not just the engine block. It's the size of the entire car that needs to accommodate it. Heavier suspension, bigger mounts, more steel in the front subframe, wider track width, bigger car.