Why modern diesels are so good.

This is a little something from the archives, a conversation with Will from PPC magazine tackling some common diesel questions.

1 What’s that bloody horrible noise?

Time to start using big words. The fundamental difference in combustion between petrol and diesel is that in a petrol engine the mixture is prepared in the intake then breathed in through the inlet valve as a ready to burn mixture. In a diesel only air goes in through the inlet, it is then compressed in the chamber (sometimes reaching over 600°C), then diesel is injected into the chamber and ignites in a stream of fire and takes most of the power stroke to burn.
At idle, however, only a little bit of fuel goes in, there is a short delay whilst it soaks up the heat then it goes bang all at once in a very short time. This makes a much sharper rise and fall in pressure and a harsher noise than an equivalent petrol engine.

A Diesel engine earlier today.

But you might be surprised to hear that at full load, there is very little difference in combustion noise between petrol and diesel and some times a diesel can even be quieter!
But the most noise you hear from conventional diesels is not from the combustion at all, oh no, its from the fuel injection pump and injectors. The massive pressures needed to squirt a fine mist of fuel into a compressed cylinder of hot air require a powerful pump which has its own pistons thumping up and down. A twenty year old design may thump the fuel pressure up to 250 Bar every stroke.
When testing the pumps on a rig we have to wear ear defenders, its that loud. There are times during a cars development when we need to measure just the combustion noise and then we have to wrap the pump, high pressure pipes and injectors in lead sheet to keep them quiet.
Funny old world.

Things are very different now, the current generation of diesels use multiple injections in each cylinder even to smooth out the pressure rise and fall during combustion in a way that Rudolf Diesel could only have dreamed of, coupled with three piston pumps which provide a smooth regulated fuel pressure and are inherently quieter.

2 Please tell me about LPG and diesel.

It’s a good thing.
No, really, it is.
The effect is very similar to putting Nitrous on a petrol engine but with improved fuel economy too.
Injecting up to 40% LPG into a diesel engine does a number of things, the main one being that it helps the flame to propagate through the whole mixture and actually increases the efficiency of the diesel fuel (more molecules get burnt in the chamber). It also has energy of its own to contribute.
About 5% at idle will quieten combustion noise, but at part load cruising conditions you can make up to 40% of the total fuel going in LPG which boosts combined fuel economy by more than 10%.
It also has the effect of effectively illuminating turbo lag.
The down side is that unless the system is calibrated properly to your application then you risk the engine pinking and associated damage.
I did develop a simple mechanical system years ago, before going digital, but it is impossible to get the fuelling right under all conditions and engine damage is almost inevitable.
If you use a well mapped digital system then the benefits are massive, huge power increase (one of my first attempts was with a Sprinter van which went from the standard 110bhp to 195bhp and masses of mid range torque) and it gets cheaper to run.
But beware, if you run out of diesel at high loads and try to press on with just LPG then the engine will pink like hell and almost certainly will melt. Don’t do it.

3 My understanding of common rail injection is that, unlike the older
systems with a pump that squirted fuel into each cylinder in turn,
common rail has a similar architecture to petrol EFi systems. A high
pressure pump feeds a common fuel rail and the injectors are fired
electronically (on new systems) or by a lobe on the cam on older ones.
Have I got this right?

Pretty much, the fuel rail pressure is in the region of 2000 Bar on some engines, a Piezoelectric crystal is used instead of a solenoid for the injectors, they run at up to 160 Volts and move a very small amount but allow us to control up to 5 individual injections for each combustion cycle. A small amount goes in to start warming things up and start a gentle pressure rise, then a slightly longer one etc. then the main body of fuel is injected, allowed to combust and then a few more small injections just to help finish off the combustion, help emission control and allow the pressure to drop gently.
This amazingly detailed control of the combustion process allows the engine to run at its best potential under the full range of driving conditions. This in turn allowed the valve train and ports to be optimised for air flow rather than older engines which used restrictive high swirl designs to improve combustion. And this allowed for greater turbo control, we could now muck about with injection timing and volume to tweak the flow of exhaust gas, helping to reduce turbo lag, which coupled with running higher boost pressure using better turbos that have a greater flow range give the engine a much wider torque band, pulling strongly from low revs right up to the red line. All these things together are the main reason modern diesels are so powerful and yet still economical.

Just to complete the historical picture some engines used Unit Injectors which are slightly different, they have an individual piston pump for each cylinder, running from an extra cam shaft. A solenoid is used to open and close a bypass port which lets the fuel escape back to the tank. If the piston is pumping and the port is closed then the pressure rises until the injector needle is forced off its seat and injection commences. It was the best technology until Piezo injectors were invented.

4 Why won’t Diesels rev? Is it because the fuel won’t burn quickly enough?

Every fuel has a certain burn speed, petrol is about 20 to 40 meters per second (40 to 80mph) depending on mixture strength, exact fuel composition and about a million other factors. Diesel burns about 18% slower, ish. So if the engine goes too fast then the mixture is still burning when the exhaust cycle starts which is pointless.
Another factor is that older diesels needed lots of turbulence in the chamber to mix the fuel spray jet with the air and so had very high swirl inlet ports which can be restrictive at high speeds.

5 Why are they inherently more fuel-efficient? Is it simply because of
the much higher Compression Ratio?

The Compression Ratio is indeed a big part of this (though technically it’s the expansion ratio that’s important for good thermal efficiency), but there are also a number of other important factors too.
A simple diesel has no throttle, so there are no losses from dragging air past a blockage at part load, unlike petrol engines.
Then there is the fact that it always runs with a very lean mixture. The cylinder fills with the same amount of air no matter how much welly you give it, your accelerator pedal only changes the quantity of fuel injected. Because the fuel is injected directly in to the chamber it spreads from the injector nozzle and mixes with more and more air until it reaches the right mixture strength to combust, the exact process is a lot more complicated than that but you get the idea.
The improved efficiency is most pronounced at part load, cruising gently and driving round town. At full chat there is much less of a difference between the two engines. If you measure efficiency as grams of fuel used per kW hour of energy (just like on you electricity meter on your house) you will find that at best petrol engines can get down to 250g/Kwh and diesels manage to get down to 240, for cars that is, not a huge difference.
Last but not least is the fact that although diesel fuel has about the same energy content as petrol at 42.5 Mj/Kg (that’s mega joules of energy per kg), is a little bit denser at 0.85kg per litre as opposed to 0.75kg/l for petrol, so because we buy fuel by the litre and not by the Kg you get better value for money (energy/litre) from diesel.
All these factors add up.

6 Know anything about 2-stroke diesels? I used to go to school on a bus
that was ancient even then (this was 1978). I think it had a Comer
engine but either way it was a 2-stroke supercharged (I think) diesel.
I think it had opposing pistons and was all very weird. But it sounded
good at full chat.

The Comer engine is just stunning, find a cut away diagram and just marvel at it.
Two stroke’s need some sort of pump to force the air into the inlet port, older style little bike engines use the crank case to do this, as the piston goes down it compresses the mixture in the crank case then unveils the inlet port so it all rushes in, the problem is you get all the crank case gas and oil burning in the engine, hence the trail of smoke.
Big two strokes tend to use a separate pump (supercharger) to do the job so they don’t have the smoking habit.
The comer had three cylinders lying on the floor; each cylinder had a piston at each end connected to a bell crank type con rod to a central crank shaft mounted above the cylinders.
Madness but elegant and a very low centre of graffiti.
Other opposed piston two stroke diesels include the Rolls Royce K range multi fuel units fitted to 432 armoured personnel carriers and Scorpion ‘tanks’ plus the wonderful Napier Deltic locomotive engine which is like a diesel Toblerone.
Two stroke engine produce power on every down stroke as opposed to every other down stroke on a four stroke engine, so you get more power for a given size or weight of engine. This makes them very popular for high power applications, Detroit Diesel make a splendid range of supercharged two stroke diesels and sound absolutely mad!
Some times a turbo is added as well, but there still has to be a supercharger or else you could never get inlet pressure to get the engine started.

I hope this gives a little illumination, as with most things the truth of the matter is rather complex and the above replies are massive simplifications, but a little knowledge is a dangerous thing, and after all dangerous is how we like to be.

Leave a Reply

Your email address will not be published. Required fields are marked *