One of the great problems with a conventional car engine is that most of the fuel’s energy gets thrown away through the exhaust (about 30%) and radiator (another 30%ish), in fact your car engine is lucky to extract 33% of the energy from the fuel, and that’s only at full load, at low loads your petrol engine will be below 10% efficient! So wouldn’t it be great if there was a way of trapping all that heat energy and getting more out of it.
Enter stage left the Sterling engine, invented by an eccentric Scottish vicar in 1823 and now used in many combined heat and power systems for large buildings.
The name is now applied to a whole range of ‘hot air’ engines – the idea is that you have two pistons and cylinders that are connected in some way. A hot cylinder heats up the gas inside it, causing expansion, the gas is then pumped to a cold cylinder and contracts, thus pulling that piston up. As the heat energy can be totally used up by the engine, in theory it could be 100% efficient. Sounds simple doesn’t it.
Small Sterling engines have been made that work on a temperature difference of only a few degrees, so holding it in the palm of your hand makes it work! The trouble is that they are not very powerful for their size because you need a lot of surface area to transfer the heat energy into the working gas in the cylinders. This gas, often helium because of its excellent heat transfer ability, is trapped permanently in the engine. The more gas you have the more energy can be pumped round, improving efficiency, but this requires high gas pressures and very good piston seals which has been the downfall of some optimistic designs over the years.
Because the fuel is burnt outside the cylinders, this is called an external combustion engine, same as steam engines. And as the combustion is continuous, there is very little noise, almost silent. And they can work on pretty much any fuel, petrol, diesel, coal, chip fat or even junk mail.
It’s unfortunate that in practice, complete heat transfer never happens so the total fuel efficiency ends up being only 10% better than a conventional internal combustion engine. But the main benefit comes when the two types of engine are used together, the exhaust and coolant from the internal combustion engine being used to run the Sterling engine, together extracting up to 70% of the fuel’s energy. Obviously this ends up being a big, complicated heavy lump, but that hasn’t stopped people putting it into cars.
A simple version of a Sterling engine has two cylinders driven 90º apart, one cylinder is heated up and the other is cold. The two cylinders are filled with your working gas, possibly helium, and joined by a big tube which contains a regenerator. That is a posh word for a sort of radiator, or heat exchanger, that keeps the heat on the hot side of the engine and the chill on the cold side, this is a very important part and has a massive effect on the engine’s efficiency, a good one will recover 95% of the heat energy.
You may be surprised to hear that one of the most successful Sterling engines was made by Phillips, better known as purveyors of electric shavers and expensive tellys. The Dutch company of boffins started on the design in 1938 but were delayed by an inconvenient world war. The work lead to a very nice portable generator set that enjoyed modest sales success, but the focus of my ramblings this month is the 4 cylinder engine, based on their work and made by United Sterling of Sweden, that was fitted to a Ford Pinto in 1974. The V4X31 used base engine parts from the Saab V4 and was the first car to be driven directly by a Sterling engine. It produced 115bhp at only 3500rpm and worked at about 40% efficiency.
The V4X31 was an early insight for Ford who were working with Philips on a 4 cylinder 170 bhp swash plate engine, that’s where the crank is replaced by a tilted disk, as each piston rod pushes down on the disk; pushing it round in a circle.
Cunningly, in both these engines, they managed to get both the hot and cold cylinders in the same bore, the lower piston having a hole in the middle for the con rod of the top piston. At this point you really need to look at my hastily crayoned diagram, because I am now going to say ‘rhomboid drive’!
Apart from being a great phrase to baffle the pub expert with, rhomboid drive is the way they managed to get both con rods to move up and down with absolutely no side movement, essential when one con rod goes through the middle of a piston, hopefully without any gas leaking out of the hole.
The system uses two cranks in parallel which are geared together at the bell housing end of the block. Each piston has a fixed con rod dropping down to a joint where two articulated con rods are attached, one going to each crank. Who ever thought that one up probably didn’t get out much.
To stop the gas leaking out, which was nitrogen at 150 Bar, the piston seals were nylon sleeves that rolled up and down, a little like rolling nylon stockings down, funny lot the Dutch. It also had the added benefit of having virtually no friction.
Of course the home mechanic could possibly try constructing a simple sterling engine by converting a V twin bike engine to run with one cylinder being heated from the coolant or exhaust from a conventional engine, the other cylinder being water cooled. You could remove the valves from the heads and connect both inlet ports together with the pipe containing the regenerator, which could be an old intercooler cut down, and something similar nailed on to the exhaust ports too. Then belt drive it onto the conventional engine’s front pulley.
It wouldn’t be light but on something like an old Range Rover it could bring the fuel economy up to diesel levels and deliver something like another 50bhp for free. Which is nice.