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Thermal Isolation Head Gasket

Because only the cylinder head is really touchy about temperature
  [vote for,

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

bs0u0155, Apr 27 2017

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]


       // thermally isolating gasket //   

       Been around since the 1980's in some diesel engines.   

       The block temperature limit is more to do with thermal expansion of the bores, and the heat degradation of the lubricants.
8th of 7, Apr 27 2017

       //expansion of the bores, and the heat degradation of the lubricants.//   

       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?
bs0u0155, Apr 27 2017

       Mumble mumble, Carnot cycle, mumble, Gibbs free energy, mutter ...   

       The piston/cylinder is a sliding metal-on-metal contact. If it is not lubricated, Bad Things come to pass.   

       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.   

       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.   

       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.
8th of 7, Apr 27 2017

       waiting for the </link> (html) and I thought combustion motors are a thing of the past.
pashute, Apr 30 2017

       //The piston/cylinder is a sliding metal-on-metal contact. If it is not lubricated, Bad Things come to pass.//   

       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.   

       //So, either the piston and cylinder have to be cooled, or the lubricant has to operate effectively at a much higher temperature.//   

       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.   

       //Ceramics have been tried as a substitute for metals, but no satisfactory solution has been produced as yet.//   

       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.   

       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?
bs0u0155, May 01 2017

       //I thought combustion motors are a thing of the past.//   

       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.   

       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.   

       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.
bs0u0155, May 01 2017

       //While electricity is still made from oil & gas//   

       Here comes the sun, little darling ...   

       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.   

       So, I'm with [pashute] (that is, living near the edge of a well-insolated desert).
pertinax, May 02 2017

       <ponders the lack of availability of plug-in hybrid aircraft>   

       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.
8th of 7, May 02 2017

       //cost of manufacture//   

       I realise that. That's why I'm not buying one now, but only when my existing car is no longer fit for purpose.
pertinax, May 02 2017

       //my existing car is no longer fit for purpose.//   

       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.
bs0u0155, May 02 2017


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