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The cooling of an engine block is a seemingly simple affair. Pump
water in the bottom, it flows up the sides of the cylinders through
some channels in the cylinder head out to the radiator and back.
Lovely. Except it’s the wrong way around.
The cylinder head, specifically the combustion chamber
is the
part of where the majority of the heat originates, and its the part
that gets really touchy about being too hot. Exhaust valves can
melt, while intake air may cool the intake portion of the chamber
enough to create huge thermal gradients over a few mm. Should
the head get too hot, it can damage itself through expansion and
warping or hot spots can develop igniting the fuel/air mix
prematurely causing detonation/pinging.
Because of this, the bottom up coolant flow is a little stupid, but
it stuck around because that's how it had been since forever,
specifically since before the water was pumped and
thermosyphoning was all that was required <link>. Many modern
engines flow coolant the other way. The GM LT1 used head-then
block flow as one of the innovations that gave it 20% more power,
better torque and fuel economy than the 350 predecessor. The
other issue is that any boiling coolant tends to rise against the
flow, filling the head with steam, rather than lovely water, so
there's that to deal with, usually using a small orifice at the top
and return line to the reservoir.
So, reverse flow delivers maximum cooling power to the head and
the block sort of receives the leftovers. Now, the block can
actually stand to operate at a higher temperature than the head,
in fact it should be more efficient for a number of reasons. As an
example, let's set the head temperature at 80C. This is a few
degrees below the usual and should allow better margin for error
regarding high compression ratios, high workload etc and peak
temperatures that form oxides of nitrogen and so on. Lets set the
block temp at 110C.
Following ignition, around TDC, combustion begins and
temperatures/pressures rise very quickly. These peak shortly, say
10 degrees, after TDC when the piston has barely moved. After
this the temp x pressure decreases as the gases expand and heat is
lost to the cylinder walls and piston. By hyper cooling the
combustion chamber, we should be able to shave the top off the
peak temperature and NOX production is non-linear with
temperature so we get a disproportionate pay off. As the piston
descends the combusion gasses are now exposed to the cylinder
wall rather than just the combustion chamber. Here the increased
wall temperature will lead to decreased heat (and pressure in
equilibrium) transfer to the walls. As such the rate of
temp/pressure fall will decrease leading to increased average
pressure on the piston, which translates to torque.
The block will also gain proportionately less energy, some will go
to mechanical work, most will go to increased exhaust
temperatures*. This means less energy to dissipate from the block
to the atmosphere and much better temperature gradient to do it
with. The biggest flow of energy is likely to be block>head via the
head gasket/bolts.
This is where the insulated head gasket comes in. A thin, pure
copper racing gasket would happily allow rapid thermal transfer
along the 30C gradient from block to head. Instead, lets put a
thermally isolating gasket made out of a ceramic fiber.
There, a nice efficiency & smoothness boosting dual zone cooling
system. It would pair well with the ceramic thermal barrier
coatings some are applying to piston/valve/combustion chamber
surfaces to further reduce heat transfer.
A clever progression would be to avoid the head>block water flow
completely and have fewer holes in the mating surfaces. Instead
have the coolant flow into the head, leave at 80C through the
side. The block coolant can then leave at 110C and interface with
the head coolant via a heat exchanger, loosing its heat to the head
efflux. The 110C head efflux then routes to the radiator.
Judicious use of extra "thermostats" at different opening
temperatures could be used to isolate the head from the radiator
circuit in the usual manner, and the block from the head circuit at
a higher temperature to speed warm up.
*good for catalytic converters
1937 Thermosyphon cooled engine
https://upload.wiki..._Maintenance%29.jpg
[bs0u0155, Apr 27 2017]
Small piston movement at TDC
https://upload.wiki...f_Piston_Motion.png
[bs0u0155, Apr 27 2017]
Temp vs thermal losses, Jump to fig 19
http://www.scienced...i/S0306261917301022
[bs0u0155, Apr 27 2017]
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Annotation:
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// thermally isolating gasket // |
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Been around since the 1980's in some diesel engines. |
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The block temperature limit is more to do with thermal expansion of the bores, and the heat degradation of the lubricants. |
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//expansion of the bores, and the heat degradation of the
lubricants.// |
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But I want to make it hotter!! wait, are diesels making their
blocks hot just from the compression heating? I guess they
get nothing but positives from increased head temperature? |
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Mumble mumble, Carnot cycle, mumble, Gibbs free energy, mutter ... |
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The piston/cylinder is a sliding metal-on-metal contact. If it is not lubricated, Bad Things come to pass. |
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So, either the piston and cylinder have to be cooled, or the lubricant has to operate effectively at a much higher temperature. Ceramics have been tried as a substitute for metals, but no satisfactory solution has been produced as yet. |
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As you point out, over-hot valves have an annoying tendency to liquefy. This also falls into the category of being a Bad Thing, and because of their geometry, they are difficult to cool. |
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The answer might be a four-stroke engine with the equivalent of Schnürle porting, giving a flat head and eliminating the valves. Forced induction is required, though. |
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waiting for the </link> (html) and I thought combustion
motors are a thing of the past. |
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//The piston/cylinder is a sliding metal-on-metal
contact. If it is not lubricated, Bad Things come to
pass.// |
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Yep. Nothing to see here though, I'm operating the
block a few degrees hotter, in the general range of the
temperature that the oil operates anyway. Standard
synthetics cope very easily with this temperature range.
Fully synthetics are designed to be OK in the extremes,
such as a very low flow around hot turbo bearings. |
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//So, either the piston and cylinder have to be cooled,
or the lubricant has to operate effectively at a much
higher temperature.// |
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Everything is still cooled in the conventional way. I'm
hedging on the side of safety in the head. I'm playing
fast and loose with the block, because there is a lot
more margin for error. I'm wondering what the main
source of heat in the typical block actually is. Heat
transfer through iron cylinder liners from cooler/lower
pressure and less turbulent gasses can't be tremendous,
although the exposure time is greater. |
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//Ceramics have been tried as a substitute for metals,
but no satisfactory solution has been produced as
yet.// |
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The dream of the 2000C all ceramic engine with
stunning efficiency is dead. Those temps were always a
dream, the thermal expansion alone disqualifies it for
practical applications, but the combustion chamber
temperatures are the real killer. Peak combustion
chamber temperatures are what govern NOx emissions
and we can't be having those. |
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Ceramic coatings have promise I think. There are a
number of thermal barrier coatings (TBC) available as
well as oil shedding coatings and nice low friction
coatings. I haven't seen a lot of data however. The idea
is that you can ceramic coat the piston/valves/chamber
walls to absorb less heat. It's not clear what that means
in practice though. If I coat a piston with an insulator,
it's true that there will be less transfer from combustion
gasses through piston to say, oil. But will the surface of
that coating experience higher temperatures? does that
matter? |
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//I thought combustion motors are a thing of the
past.// |
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I think the transition will take a long time. As
technology progresses, engines will improve. Probably
faster than electric motors. In fact, for electric cars the
actual motor is almost unimprovable. There's a lot of
scope for improvement in the internal combustion
engine that will make it something of a moving goalpost
for the electric systems to beat. Clever valve technology
should be able to vary lift/duration almost infinitely to
the point where two-four stroke transition is possible.
Management of thermal issues like this idea could
scrape an additional percent or two on efficiency. |
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Fundamentally though, it's hard to beat the power
density of the fuel-engine system. You can get 40kW for
an hour out of a few gallons of fuel and a 2 stroke dirt
bike engine, and pick it all up with one hand. |
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While electricity is still made from oil & gas, there are
going to be many occasions where a 30% efficient gas
engine is a lot better than the electric equivalent.
While there are some power plants operating above 50%
thermal efficiency, not many are. You always loose 5%
in electricity transmission and distribution. Tesla claim
their charging is 92%, users report 80% real world
efficiency. The efficiency of getting that power out is
very difficult to find. Lets ball park at 10%. So a Tesla
gets 90% of 80% of 95% of 50%, or about 35%. Right at
35%. Exactly where regular cars are. |
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//While electricity is still made from oil & gas// |
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Here comes the sun, little darling ... |
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Barring the unforeseen, my next car will be a pluggable hybrid. In winter, it will burn some fuel (whether internally, or from a power station). In summer, except for 40-odd litres for the family holiday, it probably won't. |
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So, I'm with [pashute] (that is, living near the edge of a well-insolated desert). |
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<ponders the lack of availability of plug-in hybrid aircraft> |
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Stirling engines have the intrinsic problem that some of their components need to have conflicting physical properties, principally at the "hot" end of the working chamber, to achieve high efficiency, and even then the power-to-weight ratio is poor, making them unsuited to anything other than static applications. |
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I realise that. That's why I'm not buying one now, but only when my existing car is no longer fit for purpose. |
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//my existing car is no longer fit for purpose.// |
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It can be made fit for purpose effectively forever.
Perhaps not economically. For environmental
reasons I am going to need a variety of older cars,
with a variety be of older big block V8 engines and
a variety of superchargers. |
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