Roothy explains how diesel engines work

After more than 40 years, Roothy’s still learning a thing or two about diesel donks.

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OKAY, now if this gets too simple for you, remember I’m writing it for me. And I’m pretty simple when it comes to high-tech stuff, so hang on to your hammer and let’s take a look at how diesel engines work.

I’ve got a feeling Editor Matt will be laughing right about now. As an old mechanic who still likes to play with the tools he knows all this stuff, but I figure there’ll be a few people out there who could use a refresher. And to be honest, even though I’ve been playing with diesels for 40 years, I’m still learning things. All it takes is to hang out with the experts for a day or two.

So last month we pulled the engine out of a 61 Series Toyota to get a sample of the last of the mechanical-injection turbo-diesels Toyota produced. Some people reckon it’s the best motor the factory ever built, but I’ll reserve my decision on that until Milo 2 has done her first million kilometres or so.

I’m doing what I did with the original Milo, mostly because 10 years and 500,000km later it’s still working really well.

The engine, gearbox and transfer have been trucked down to Melbourne to Terrain Tamer for a full rebuild. Terrain Tamer’s ‘exchange’ units use many parts developed over the years that are stronger than stock to start with. You get that when people all over the country are sending you their tired and broken dungers – lots of experience at the pit face seeing what works and what needs improving.

Roothy engine buildBut before the motor left the Sunshine Coast we plucked out two vital components: the fuel pump and the turbocharger. These two bits are critical to a diesel’s performance for a whole lot of reasons. And just like the last time a decade ago, I’m taking them over to MTQ Engine Systems, Australia’s largest diesel fuel-injection specialists. This, by the way, is what pretty much every diesel workshop in the country does with these components – takes them to a specialist.

Before understanding why the fuel pump is so critical we probably need to recap how a diesel works. In a petrol motor, the fuel air mix is exploded by a spark. In a diesel, it’s exploded by the high temperatures air can achieve when it’s compressed. That’s why we need things like glow plugs, or pre-warmers, to get this temperature cycle working before the pistons romp up that cylinder and squeeze the living daylights out of the air. Then right when the air has been squeezed so much it’s hot enough to explode something, the injector cracks open with a blast of fuel.

That happens just before ‘top dead centre’ of the compression stroke which, just like a big leg on a bicycle pedal, means the biggest part of the energy is expended forcing that piston down again. It twists the crank… and you know the rest because you let the clutch out and the whole plot shot off down the road in a series of explosions that are almost too amazing to comprehend.

In order for the injector to squirt fuel into an already highly compressed and hot environment, the fuel pump has to develop incredible pressure. This is typically at least 15,000psi, roughly 500 times the pressure of a bicycle tyre. That sort of pressure is enough to blast diesel straight through your skin, which is why you never muck around with diesel fuel lines when they’re running. The old ‘pluck lead and see which one’s not firing’ thing that works so well for us bush mechanics on petrol engines just doesn’t cut the mustard on diesels.

Milo 2’s 12HT motor features an inline injection pump, which these days is only seen on heavy industrial engines, rural plant-like harvesters and tractors, and big trucks. The ‘small’ diesel world has been overtaken by rotary pumps and common rail-injection which allow much greater power and efficiency at higher engine revs. A lot of that has to do with economy, not just the economy of fuel consumption but the economies of saving a few quid at the production level – replacing expensive mechanical bits with computers and all that jazz.

Roothy engine build 2
And you get to see why when you look inside a mechanical inline-injection pump. Not only does it look like a mini six-cylinder inline engine with a cam shaft driving a series of ‘pistons’, it also has to add or subtract fuel to each squirt – oh, how technical the Rooth does wax – depending on whether we’re climbing that hill or coasting down the other side. As each piston has a constant stroke volume, varying fuel delivery is achieved by rotating the cylinders themselves to vary their injection volume.

Don’t worry if that doesn’t make sense. What does make sense is when you see the governor rod moving backwards and forwards twisting all the cylinders at the same rate to achieve some sort of consistency. Then you spend another hour or so playing with the governor and marvelling at how it works.

Right, have we learnt anything? I have, if the pump plays up take it to MTQ and pick it up a day later. And then don’t fret about how much it cost because, by crikey, there are some parts, effort and expertise required to get it right. Most of all, though, knowing how many little, precise mechanical components are rubbing themselves silly in there is just one more reason to fit a good fuel filter, use quality oil and always change the oil filter.

Lubrication is everything and is also very cheap insurance. I’ll let you know how it goes once we’ve got it all back together.


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John Rooth
John Rooth

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