Southerly by David Haywood


The Pivotal Engine

This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 4th August 2007, 5 pm - 6 pm.

You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.

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Theme music...


[Sound of engine starting]


That's the sound of the pivotal engine: a radical new two-stroke engine design from a sub-division of Mace Engineering -- one of Christchurch's largest companies.

Now when you think about two-stroke engines (if you think about them at all) you'll no doubt think about the type of motors you find on cheap lawnmowers and chainsaws: low cost, but also low efficiency, short lifetime, and very high exhaust emissions.

But two-stroke engines do have an important advantage over their more environmentally-friendly four-stroke big brothers. This is because they have a power stroke once every engine revolution -- rather than once every two engine revolutions as with a four-stroke motor -- and therefore have approximately twice the power-to-weight ratio.

And it is this improved power density that has seen a number of automotive engine manufacturers conducting research into two-stroke motors over the last couple of years.

A two-stroke engine with the environmental features of a four-stroke could significantly improve overall vehicle performance. But the Christchurch engineers working on the Pivotal engine have a big head-start on practically everyone else. They've been developing their engine at full throttle for over a decade.

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I'm now sitting in the office of Paul McLachlan, the inventor of the Pivotal Engine. Can you explain how -- in terms of its basic design -- the Pivotal Engine differs from a conventional two-stroke motor?

Paul McLachlan:

We set out to overcome what are the inherent shortcomings of a conventional two-stroke engine. [In such motors] you have a sliding piston, restrained by a cylinder -- and in that cylinder you're cutting the ports for the engine to breathe, i.e. the transfer ports, and the exhaust port.

[This] causes a number of difficulties. We've overcome these by designing the engine to have a piston which is restrained by a pivot bearing, [but which] still has a connecting rod down to a crankshaft in a conventional way.

[Restraining the piston] allows us to take the load off the surface of the chamber. [This] means that we don't have to lubricate the piston as a sliding piston, and it also means that we have a pivot point on the piston -- which allows us to run water into the piston itself and back out the other side via the pivot.

So it becomes a water-cooled piston. And so we have a water-cooled engine with a water-cooled piston, and that overcomes all of the inherent shortcomings of a two-stroke engine that stops you using it [for example] in your car.


So I guess a way of explaining it would be to say that in a conventional two-stroke motor you have a cylindrical piston inside a cylindrical cylinder (as it were) sliding up and down.

But in your pivotal engine, the piston is more like a flapper. So seen from the top the 'cylinder' would be square in profile, and seen from the side the piston would be moving up and down

-- and tracing out a shape a bit like a Chesdale cheese segment.

[click here to see an animation of the Pivotal Engine].

Paul McLachlan:

Yes, and we're using only the outer part of the cheese segment. So it moves through about a 36 degree radial angle.


So when you talked about the problems with conventional two-stroke motors -- why they're not used in a normal automotive situation -- I guess you could summarize the problems by saying: low thermal efficiency, short operational life, poor emissions (particularly in terms of oil residue in the exhaust).

How does the Pivotal Engine deal with those issues -- what improvements have you been able to make?

Paul McLachlan:

We'll start with longevity. The problems with a two-stroke engine [in terms of longevity] is that if you're wanting to get the high-performance benefits of a two-stroke engine, then you end up with quite sizable ports. Therefore what you've done is taken away a lot of the physical support for your piston by taking sections out of the cylinder wall.

[But this] also has the effect of reducing the amount of contact which would take heat out of your piston. So we overcome the restraining by the fact that it's held by bearings; and the heat by the fact that we're internally water-cooling the piston.

The emissions are really a matter of two things. There's the lost unburnt fuel with a normal [carburetted] two-stroke -- whereby you lose [fuel] out the exhaust totally unburnt. This decreases the efficiency of the engine, but also results in a very unacceptable level of hydrocarbon emissions.

Now that gets overcome in any two-stroke engine to a very high degree by having a direct injection system -- which means to say you're injecting the fuel from the top of

the head, and only after you've closed the exhaust ports, so that you're not losing any of that fuel.

The other [source of] hydrocarbons is the oil you use to lubricate the piston. We overcome that by the fact that the piston is now not actually in contact with the surface of the chamber, and therefore we only need a very small amount of oil [which is directed to] the compression seals.


So how much oil does the Pivotal Engine use in comparison to a conventional two-stroke?

Paul McLachlan:

At this stage we would say that it's around about a tenth of the amount of oil that you would use in a modern direct-injected type of two-stroke engine.


... and normally you'd say they use about ten times the amount of oil of a good four-stroke -- so you're using about the same amount of oil?

Paul McLachlan:

We're getting close to being able to having comparable oil usage to a four-stroke engine. But on top of that, of course, you don't have to have an oil change -- so the overall use of oil is actually slightly lower.


In which areas do you see the Pivotal Engine first making an entry into the market? You've been going for over a decade now with some pretty intensive R & D -- so I imagine that coming to market is something you're pretty keen on?

Paul McLachlan:

Yes, we have in the last two or three years been focussing very much on a first step into the market place. We [initially looked] at light aircraft, and we have a lot of interest from that point of view, but there were various reasons why we decided on a generator product -- we have a good partner in the US for

a military generator, and that's our main focus at the moment.

The potential there, of course, is based on its power density -- and so it's a stand-alone portable generator. We have simulated all of the components that make up the stand-alone generator for using our engine, and we end up with a weight which is about 25 per cent of the weight of a [comparable military] generator. So for many applications that will make it an ideal choice.

And there are a lot of other opportunities once we have this first product as a base engine. Certainly we have a lot of interest in the hydrogen potential. We have full control of all of the thermal surfaces on the combustion chamber, and that's due to the fact that we have the piston cooled independently with its own water cooling.

In hydrogen the big advantage is that we actually can run the engine at a reasonable temperature for good thermal efficiency -- but we can avoid any hot-spots on the piston or in the head, [which is] the limiting factor in trying to run hydrogen in a conventional engine.

We see a vehicle such as a plug-in electric vehicle -- which would be a hybrid with hydrogen as the core range-extender in the vehicle -- and we have a lot of interest in that concept.


In terms of your initial market niche that you're aiming at -- the generators -- do you have anything like a timeline for when you expect the first production to roll off the line?

Paul McLachlan:

We have component supplier already making the key components. We're hoping to have most of those ready at the beginning of next year to assemble engines that are representative of a series-production engine. They will go into test

with the generators attached at our partner on the generator project -- and also those engines will then get used in some hydrogen development programmes, which we're working on with some European development companies.

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It's heartening to see that a device as astonishingly simple as the piston-cylinder mechanism in an internal combustion engine can still be improved by a cunning engineer. The ingenious Pivotal engine avoids the efficiency, longevity, and emissions problems that plague conventional two-stoke motors, while still delivering a mind-bending power density at low cost.

Its debut as a military generator will be closely watched, and it will be interesting to see where it ends up being used in the future.

Theme music...

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Further information on the Pivotal Engine:

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