Theses are my thoughts on the question. I can't find an answer to "why not?". It there something I am missing, or are auto manufacturers simply too focussed on cars only?

The automotive industry has been developing internal combustion engines for many decades. They are now routinely capable of running for 20,000 miles service interval, which is a simple oil and filters change.

20,000 miles will be around 2000 litres of fuel, which is comparable to the oil required to heat a house in the UK for a year using an oil boiler.

Air source heat pumps are only just capable of offering cost savings to a UK household, compared to an oil-fired boiler, because of the inefficiency of the power stations and electricity grid in delivering electricity to the house, compared to burning the fuel locally. A local heat-pump gain of 4+ has to be multiplied by electricity generation and distribution inefficiency (around 0.5? Not sure).

The auto manufacturers' investment in internal combustion engines in under threat from electric vehicles. They have made the investment to mass-produce engines, and ought to welcome another possible use for much the same.

So, why don't the auto manufacturers develop an domestic air source heat pump based on an auto engine?

The water cooling system would be the house's central heating system. Auto cooling normally runs at 90C. Domestic heating systems normally run at no more than 70C for safety reasons, and often as low as 30C for thermodynamic efficiency reasons. Would this be a problem?

I don't know what fraction of a car's fuel is lost as heat via the exhaust pipe. In any case, some (large?) paer if this could be reclaimed via a heat-exchanger, as with a condenser boiler.

The rotary motion the engine generates could either directly run a compressor, or run a generator to provide electrical energy for the house with excess sold back to the electricity utility. Unlike solar panels, this energy would almost by definition be generated at times of maximum demand (in the UK, the coldest weather. In hot countries, the hottest weather, for cooling). So this would automatically be a benefit to utility companies and they might offer a good price even without the government forcing their hands.

An auto engine can provide a range of power levels, so the system could easily modulate its output to match demand.

I'd guess that an auto engine driving a heat pump, directly or indirectly, could have a heating efficiency of 2-3. Heat pump efficiency for driving a hot water central heating system is typically 3, so that lower figure allows for some losses. A modern boiler's efficiency is 0.85. I'm guessing that overall electricity utility plus heat pump is around 1.5

Annual servicing would be an oil and filter change. By using a larger oil supply and larger filters, it might be straightforward to push this to 2, even 3 years. Costs not a lot more than oil boiler servicing. Also a fixed installation should be a more benign environment than a moving vehicle.

I know that a petrol engine can also run on natural gas. Presumably a diesel could be modified to run on kerosene. If not, this would require a household without a gas supply to use diesel as heating fuel. Governments might worry about leakage of tax revenue from the heating oil tank into a car's fuel tank. However, in any case, electric cars are going to destroy that revenue in coming decades. (In the UK, farmers use low-tax "Red diesel" for tractors and heating. This fuel is dyed red so auto fuel tax evasion can be detected).


4 Answers 4

  1. Automotive engines have their own inefficiencies. If the engine runs at its optimum speed, expect to convert ~30% of the energy in the fuel to motive power, 30% to heat in the cooling system and 30% to hot gases in the exhaust. At low loads, a petrol engine gets much worse figures, diesel scales a bit better.
  2. Reliability is not high enough. 20,000 miles takes about 500 hours, so you'd need to service the engine every month.
  3. Car engines are a bit too large for this application. A 75-kW engine produces ~70 kW of electricity (50 times as much as you need) and 75 kW of heat (a central heating furnace produces 25-40 kW of heat and only needs a small duty cycle), so you have to store the excess and run the engine intermittently. You can make the engine smaller, but that also makes it less efficient and noisier.
  4. "The auto manufacturers' investment in internal combustion engines in under threat from electric vehicles", yes, and auto manufacturers are switching for three reasons: we're running out of fossil fuels, we can't go on producing as much CO2 as we do now and we've noticed exhaust products are bad for your health. All of these would count just as much for domestic use of ICEs, so any attempt to introduce this would face heavy legislation.
  5. It's very difficult to reduce the noise of an ICE to levels acceptable for domestic use. Car engines produce ~ 80 dB running at their most efficient point, you want the noise level outside the enclosure to be below 30 dB. 50 dB reduction requires heavy, expensive and large noise insulation.
  • $\begingroup$ Re efficiency, using your figures and existing air source heat pump COP of 4.5, get 0.3 x 4.5 + 0.3 out for 1 in, so 1.65 compared to burning the same fuel in a boiler 0.85. I'd expect to raise that 4.5 a bit because the exhaust warms the source air and reduces the heat pump delta-T. $\endgroup$
    – nigel222
    Sep 5, 2018 at 17:49
  • $\begingroup$ Re engine size: fixed install can have large heavy flywheel, so 2 cylinders OK, half power output. Thought engines were efficient at a small fraction of peak power? You don't need anything like 75kw for cruising at 30mph urban in a small car. $\endgroup$
    – nigel222
    Sep 5, 2018 at 17:55
  • $\begingroup$ Re fossil fuels: until the electricity grid supply is mostly-renewable, 1 unit electricity into a house burns more than one unit of fuel at the power station. In contrast an IC engine driven heat pump will burn 1/1.65 units of fuel based on above estimate. So an improvement. A gas- or oil-boiler converts 0.85 of its fuel into heating. $\endgroup$
    – nigel222
    Sep 5, 2018 at 18:02
  • $\begingroup$ Re noise: this may be the problem. However, air-source heat-pumps are 40+ dB (like air-conditioners). Also heavy sound-damping is not a problem for a fixed install. Neither is big, within reason. It doesn't have to fit in a car. $\endgroup$
    – nigel222
    Sep 5, 2018 at 18:06
  • $\begingroup$ ICEs are most efficient at peak torque, hence the rise of turbocharged units that can have their peak torque at low rpm. If you're going to compare transmission losses you also have to account for the energy used to crack crude oil into usable diesel, and the energy to transport it etc. The vast combined-cycle steam plant at a power station is more efficient than any ICE (55+%). And the dB scale is logarithmic, a 3dB increase doubles the noise level. From 40 to 80 dB is huge. To get better than 30 db reduction, you need to enclose the engine in concrete. $\endgroup$
    – Hobbes
    Sep 5, 2018 at 19:06

I had a similar thought some time ago, but my concept was to use an internal combustion engine that was modified to use the domestic gas supply.

The use of domestic gas avoids issues with transporting and storing petrol (gasoline) to domestic premises and associated safety issues. Domestic gas is already supplied to most dwellings in certain regions, so the fuel is already there. All that is required to attach a pipe from the domestic gas supply to the engine.

Having stated that, there are a number of reasons why using an internal combustion engine as a domestic energy supply is problematic:

  1. The installation of the system would require skilled people, which could be expensive.
  2. Car manufacturers do not make combustion engines for the domestic market most likely because the market size would be small and there would most likely would be regulations that would make it difficult.
  3. Neighbors could be easily upset by both noise and exhaust fumes. Noise can be mitigated by enclosing the engine in a sound proofed enclosure - such as a shed. The exhaust fumes may require a very tall flue, which may not be approved by local municipal councils.
  4. In some jurisdictions, generating electricity requires permits and has government regulations associated with it; which can be an issue for some people with solar panels.
  5. Such systems would require periodic safety checks and the upkeep of maintenance.
  • $\begingroup$ Fred, wouldn't the efficiency comparison between an internal combustion engine and an electric motor also come into play here? Gasoline or diesel engines are notoriously low efficiency, while electric motors are quite efficient. $\endgroup$
    – fred_dot_u
    Sep 4, 2018 at 14:54
  • $\begingroup$ Efficiency is differently defined in this application. The engine's "waste heat" is not waste at all, if it ends up in the domestic heating system. All that is normally dumped in the car radiator, is domestic heating. The rotary energy pumps heat from the environment (including exhaust gases) multiplying it by a factor of 4 to 5. Overall I'm guessing 1 unit of fuel generates 0.35(rotary power) x 4 + 0.5 (water-cooling output) = 1.9 units of heat out. Much better than the 0.85 units from burning the fuel in a condenser boiler. $\endgroup$
    – nigel222
    Sep 5, 2018 at 17:33

gas and diesel engine-driven heat pumps have been on the market for decades, for use in running cooling systems for refrigerated truck trailers and railroad cars.

they are noisy, and would require the same noise-abatement systems used to control noise in residential gas-powered electric generator installations.

something to consider here is the idea of using the waste heat of the engine as part of the useful output of the system, to provide space heat or hot water for domestic use.

You will find that Honda is actually selling an engine-powered electric generator right now that runs on natural gas and furnishes both electricity and heat. the system is powerful enough to serve the needs of a small house and fits under a kitchen counter, occupying the same space as a dishwasher.

  • $\begingroup$ A link to the Honda generator cited would be nice. $\endgroup$
    – Eric S
    Sep 4, 2018 at 19:20
  • $\begingroup$ If i had it, I would. it was on the web about 1 year ago. $\endgroup$ Sep 4, 2018 at 20:25
  • $\begingroup$ fiat published an engineering analysis of this idea 30 years ago or so. they claimed if the waste heat from the engine is captured, the overall thermodynamic efficiency of a gas-fueled electric generator for home use exceeds 90%. Honda claims >95% I think. $\endgroup$ Sep 7, 2018 at 7:22
  • $\begingroup$ It’s just that you say Honda is selling a device and don’t provide a link. If it is on the market there should be a web page somewhere. If there isn’t, you should probably edit the answer. $\endgroup$
    – Eric S
    Sep 7, 2018 at 8:15
  • $\begingroup$ Perhaps this is what you are referring to? world.honda.com/news/1998/p980312.html $\endgroup$
    – Eric S
    Sep 7, 2018 at 8:21

The shaft seals will getcha

The problem with mechanically driving a refrigerant pump is you need very good seals on the compressor input shaft.

Why is that a problem? Well, it's only a problem compared to the alternative, which is a 'hermetically sealed' unit with no moving seals - only electrical wires enter the hermetic package, and obviously they're pretty easy to hermetically seal. That's why refrigerators and home A/Cs aren't high-maintenance.

So the only sane way to approach that problem, from reliability's sake, is to dump the seals and use hermetic electric motors fed by an electric generator on the engine. That will allow the use of COTS, already-refined, manufactured at-scale, refrigeration units.

Well golly. At this point we're just running air conditioners off a generator!

Yeah, pretty much. Not exciting.

You'd have some refinements, such as inverter drive and a bit of software, so the heat pump demand follows the generator output, so the generator doesn't need to be oversized to start the heat pump, saving money there and also helping it manage smog.


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