Sterling Engines

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.

I like this design, purely because it's weird!

 

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.

Mighty Midget

The man and his dream machine

Stuart Gunn did something remarkable; he set about creating this remarkable car in a remarkable way.

He sat down and thought about it, worked through the options, made templates and jigs and then went about welding it all together in a sensible logical well engineered way. Which is remarkable when the very idea of a V8 4×4 Midget is so splendidly mad.

The basic concept was to take the drive train (gearbox, props, diffs and hubs) off a Sierra XR4x4, take a Rover V8 3.5 with a good road tune, add in the MG Midget and blend with a home made chassis and subtle body mods. The result is a beautiful looking car that is easy to drive and blisteringly quick, particularly off the line where the extra traction from the four wheel drive gives it one hell of an edge.

With the power to scare small children...

Stuart learned his craft over the years with a number of projects, which all started off a couple of decades ago as a yoof with a Morris Oxford with a jacked up rear, a flip front and side exhaust pipes. By trade Stuart is a panel beater, which shows in the skill with which the steel wheel arches are seamlessly blended in to the MG body.

looking down on creation...

The build started with his mildly tuned Midget that he had driven round for a year or two. He then measured everything up and made suitable jigs for the wishbones and chassis out of steel box, so that the final result would be spot on and match on both sides.

The chassis uses the best bits of the original MG tub, added to a box section lower chassis and tubular upper rails which hold the top wishbones and coil over dampers on.

The 4×4 system uses the Sierra front diff and so Stuart created his own unique front cross member with the diff mounting on the right hand side (on the Sierra it mounts onto the sump).

Wishbone jigs held the bush carriers in the correct place, then steel tube was cut and welded in place to join them together, that way Stuart knew the geometry would be as predicted and both sides match.

Cunning front suspension

The suspension uprights have the struts removed and a ball joint fitted in there place, these parts came from the kit car manufacturer, MK Engineering. Being a tad narrower than a Sierra, the drive shafts had to be shortened, but not by too much, only 70mm.

Springing comes from coil over AVOs all round, with adjustable spring seats and damping. The first iteration saw 150lb springs on the rear but these ended up coil bound, now it has 225s which are spot on. Damping was adjusted to give good ride quality and handling, but also to stops the mud flaps dragging when going over speed bumps.

Wing and arches hint at the potential within.

Stopping the car is taken care of by Sierra based disk brakes with EBC Green stuff pads. As yet there is no servo, this is one possible mod for the future, but for now the braking is still excellent as long as you press the pedal firmly.

When it came to fiddling with the engine, Stuart had a chat with Dave Ellis of DJE fame. Stuart is working on a tight budget and isn’t after stupid horse power figures so a package was assembled to give the 3.5 litre about 200bhp and great drivability. It has a pair of 4.6 heads which have bigger ports and valves than the old 3.5 units and are not too expensive, a DJE 210 cam for good road manners, a Webber 500 4 barrel carb and Rover electronic ignition coupled to an Accel Super Coil. The carb breathes through a filter with a cleaver mod, because of the lack of space the top of the filter housing is the bonnet, a flexible mounting lets the filter seat even when the engine twists under acceleration. Though fuel injection is on the cards for a future mod.

Relatively light weight but torquey V8

Stuart made his own exhaust manifolds from tube and then Custom Chrome Racing very kindly chromed them and made the rest of the system. Stuart has known CCR for many years and so was allowed to use their workshop to do the fiddly bits. In fact CCR even provided the steel for the chassis and wishbones as well as making the oil catch tank. The resulting exhaust has that wonderful V8 burble but is not intrusively loud, quite subtle in fact, in keeping with the theme of the car. And all with just one small CCR muffler on each side.

Various wheel arches were tried by Stuart, including ones off a Transit double axle, but in the end he again made his own which complement the understated look perfectly. To make these he took a piece of small steel tube and bent it round the tyre, then he flattened the tube and braced it to the body with more bits of tube. This made a perfectly formed skeleton which he could then make up some cardboard arch templates and offer them up until he got the look that he wanted. Once satisfied with the templates he made steel arches and welded them on to the skeleton. The result is well made and has a factory quality feel to it as well as looking the dogs danglies.

Smoothed front give nothing away

The body was finished off by removing the bumpers, fitting a natty small bumper at the rear, adding a boot wing, a small bonnet bulge and smothering the thing an a gorgeous Rover Caribbean Blue paint job.

When designing the car Stuart wanted it to be usable every day, and on a short trip round town the car proved that this goal has been well and truly achieved. The suspension soaks up the bumps well and copes with speed bumps effortlessly. Once out on the open road the thrust from the engine is never ending, pushing you into the seat and putting a grin on your face. The Toyo Proxes tyres grip well and corners are dealt with easily.

A wonderful car to drive in any conditions.

At low speeds the steering is a little heavy, using a power steering rack to get the 2.8 turns lock to lock, but as yet without the power steering bit fitted due to the lack of space in the engine bay. Electric PAS is a possibility. But as the speed builds it becomes lighter and very communicative.

A lot going on in a small place, front suspension may recieve PAS later.

All in all, this is a simply splendid car, well thought out and professionally built. Stuart has plenty of ideas for future tweaks but the basics are very well sorted out. The car would be marvellous at hillclimbes and sprints and Stuart is toying with the idea of doing a few competitions in the coming year, I for one would love to see it out there.

As a footnote, how different would history be if BL had put something like this into production back in 1972….

Stuart would like to thank the following:

Graham and Nigel at Custom Chrome.

Dave Ellis at DJE

Many friends, family and Midget And Sprite Club members for their support and encouragement.

Tech Spec:

MG Midget Mk3 1972.

Rover V8, 3.5l, lightened and balanced flywheel, Vitesse pistons, 4.6 heads, DJE 210 cam, Webber 500 4 barrel, Rover Lucas ignition and Accel coil. Approx 200bhp @ 5500rpm.

Owner fabricated 4-1 tube exhaust manifolds with Custom Chrome Racing system.

Owner fabricated spine type chassis.

Owner fabricated double wishbones all round. Fully adjustable.

Ford Sierra Xr4x4 gearbox (type 9), props, diffs, hubs, steering and brakes. Narrowed by 70mm each side.

EBC greenstuff pads.

Toyo 195/45-16 Proxes tyres on Ace alloys.

Avo coil over adjustable dampers with adjustable spring seats all round.

0-60 4.5 seconds ish.

The most powerful piston engine in the world

Big is good, it’s official.
As you hopefully know, power is generated in our car engines by burning fuel. The heat makes the pressure go up which pushes the piston down just like you foot bearing down on a bicycle pedal. It’s a nice simple theory. The trouble is that most of the heat released into the cylinder is wasted, at full throttle about a third goes into the coolant and the other third goes down the exhaust pipe. At part throttle its even worse, up to 90% of the heat is wasted. A small fraction of the exhaust energy can be recovered with a turbo but basically it’s very wasteful.
So how can we improve things? Well, to retain more of the heat energy in the exhaust we can reduce engine speed, allowing longer for the heat to be used. But how can we reduce the heat absorbed by the cylinder wall? We could use some funky materials like ceramic but that has its own problems.

An engine, look carefully in the middle there is a bloke on the third floor! (Picture - WÄRTSILÄ)

However, one fundamental scaling rule applies to all things in the universe, if you double the volume of something, the surface area only increases by about 1.4. This is good for engines because a smaller proportion of the heat is lost to the cylinder walls. It’s not so good for creatures that breathe through their skin, like spiders which can never get much bigger than your hand without collapsing in a heap.
So why not use massive engines everywhere? Of course the efficiency bonus of big engines only applies at high loads, at part throttle the massive cooling effect of a high surface area ruins the fuel economy, so in the car world we size an engine to be appropriate for the job the car is supposed to do. For instance, when cruising down the motorway most cars only need about 18 bhp, so building a small engine that has peak efficiency at that power would be great for that single job, and indeed that is what the ‘mileage marathon’ cars do, but of course there is nothing left in reserve for accelerating, so we go for something closer to a 100bhp engine in a smallish car.
But what about something bigger? The biggest vehicles produced are ships, and in particular supertankers which can be the length of a drag race track.
So the subject our adoration is the mighty Wärtsilä-Sulzer RT-flex96-C, in particular the 14 cylinder version. Its total cylinder capacity is 25480 litres, I’ll just let that sink in for a moment.
It is a two stroke diesel engine with four, very large, turbos and computer controlled poppet type exhaust valves. It gobs out 84.4Mw which is 114800 bhp at only 102 rpm, and as you probably know torque is power divided by speed, that equates to about 6 million lb/ft of torque. Which is a lot.
Fuel economy is brilliant at full load – amazingly only 171 grams of fuel per kWh, ok that might not mean too much to most people but it is about twice as good as a diesel car. But if you drop the load to 85% and best efficiency the consumption is only 163g/kWh, which with the fuel containing 42.7 Mj/kg relates to about 51.7% efficiency. But this is not the best efficiency in the piston world, I think that is the Man S80ME-C7, catchy names.
Now that's a big crank, you should see the ballencing machine! (Picture - WÄRTSILÄ)

Everything about it is big in a whole new way; each cylinder is 960mm (37.8 inches) bore by 2500mm (98.4 inches) stroke, giving 1820 litres per pot. At full tilt, 102 rpm that is, a mean piston speed of 8.5 meters per second and the average pressure in the cylinder (MEP) is about 20 times higher than atmospheric which all means that each individual cylinder produces over 7700 bhp (the same as a top fuel drag race car) whilst consuming about 160 grams of heavy smelly fuel oil each stroke.
The outside is even more impressive, they call this type of thing a ‘cathedral engine’. At nearly 14 metres tall, 28 metres long it has four stories of walk ways round it. And weighing in at 2300 tonnes its not going to fit in a car!
Even the fuel pump is impressive, it looks like a huge V8 designed by Dr Frankenstein and delivers up to 1660 gallons per hour of heated fuel oil at 1000 bar to the common rail diesel injection system. Each cylinder has three injectors which are operated independently to control the combustion flow, the way the flame moves around the chamber, resulting in no smoke even at full load. The fuel control allows the engine to run over a wider range of speeds than older generation engines, indeed on the 12 cylinder version they managed to get it to run at only 7 rpm experimentally, that’s one ‘kaboom’ every 9 seconds, which is very slow.
It uses, more or less, the usual two stroke block scavenge intake system, that’s where the underside of the piston compresses the air. But that’s as far as normality goes, here it is refined with one way valves the size of tea trays and then the air hits four very impressive turbos, each the size of a small garden shed, before going through inter-coolers the size of Portacabins and entering the cylinder via a port near its lower edge. For starting, massive electric blowers pump in air at about 30 bar.
The exhaust poppet valves are fully computer controlled, a servo uses oil at 200 bar to move the valve up and down, so the cam profile exists only as software in the virtual world of the control box and is totally variable. The exhaust valve opening is reduced at part load to keep the exhaust temperature above 150 ºC to prevent tons of sulphur from the unrefined fuel corroding the exhaust after treatment system.
The pistons are mounted solidly onto an upper con rod, called a piston rod, that has a joint at the bottom that runs in guide rails, to keep the piston rod upright all the time. It’s called a cross head and ensures that the piston has no side loading and the oil control is tight enough to give the engine a 30 year service life, and most of the time in all those years the engine will be running flat out, which is pretty amazing. The lower end of this rod connects to the con rod proper and then on to the crank, the main bearing caps have ladders on them for the service crew to walk up.
Did I mention it’s quite big?
These engines are used in container ships like the Emma Maersk, about 170 thousand tons of it which is propelled at up to 26knots and is assisted by no less than five 8000 bhp Caterpillar engines just for helping with manoeuvring.
It’s got quite a big propeller too.