Friday 28th January 2005

Diesel engines torque it up

Manek Dubash looks at the diesel's long and honourable history

---

Following the recent announcement of the anniversary of the the very first internal combustion engine, made by Belgian Etienne Lenoir, it's worth taking a look at the ugly sister of the petrol-propelled motor, the diesel.

These days, almost every carmaker has one or more diesels in its product line-up. Not just a few either -- Peugeot Citroën has just announced that it's produced its 25,000,000th engine -- a diesel. The picture shows a modern two-stroke marine diesel engine -- said to be the most powerful in the world (more below).

But how did the diesel come about?

Rudolf Diesel

Born in Paris of Bavarian parents, Rudolf Diesel (1858-1913) studied at Munich Polytechnic where he was an outstanding  mechanical engineering student. He began his career as a refrigerator engineer. For ten years he worked on various heat engines, including a solar-powered air engine.

Backed by Baron von Krupp and Machinenfabrik Augsburg Nurnberg Company in Germany, he began experimenting with a high-pressure ammonia engine. In 1892 Rudolf Diesel was issued a patent for a proposed engine that air would be compressed so much that the temperature would far exceed the ignition temperature of the fuel. In other words, no spark would be needed to ignite te mixture.

His backers provided him with engineers to help him develop an engine that would burn coal dust -- at the time, there were mountains of useless coal dust piled up in the Ruhr valley.

Experimental engines

The first experimental engine was built in 1893 and used high pressure air to blast the coal dust into the combustion chamber. While the prototype blew its cylinder head off but, four years later, Diesel produced a reasonably reliable engine. His ideas for an engine where the combustion would be carried out within the cylinder were published in 1893, one year after he applied for his first patent.

Further developments using coal dust as fuel failed. A compression ignition engine that used oil as fuel was successful and a number of manufacturers were licensed to build similar engines.

The original oil burning engines used very crude mechanical injection equipment so Rudolf Diesel again began using air blast to provide fuel atomisation as well as turbulence for improved air-fuel mixing. It was very successful and was employed in Rudolf Diesel's third engine, built in 1895. An engine very similar to those in use today, it was a four-stroke cycle with 450psi compression.

Diesel goes overboard

Rudolf Diesel's enjoyment of fame and fortune was marred by ill health, probably caused by exhausting legal battles over patent rights and unwise financial speculations. He lost a fortune and, while on a ship to England, he disappeared overboard.

In his notebook he had marked an X by that date, and speculation has it that he was pushed overboard for political reasons, as he was a vocal critic of Germany's foreign policy.

Diesels motor on...

Diesel's new engine, which his wife convinced him to name the engine after himself, was soon accepted throughout the world, and many of his engines were produced under licence.

The diesel's success can be attributed to many factors but its ability to produce prodigious amounts of torque for its capacity and its good fuel economy --which results from the fuel's greater energy-density over the more-refined petrol -- are key elements of the equation.

The most powerful diesel in the world is said to be the Wartsila-Sulzer RTA96-C turbocharged two-stroke. Designed for large container ships, it weighs 2,300 tons and pushes out 108,920hp and 102 rpm, and 5,608,312 lb-ft of torque, consuming 1,660 gallons of diesel per hour to do that. It has 14 cylinders with a total capacity of 25,480 litres.

More about this massive machine here: www.bath.ac.uk/~ccsshb/12cyl/

[MrFlibbles]

As the great Robbie Coltrane said: "Its a good job he wasn't called Rudolph Studelfunfenheimer"

[franv8]

I think you will find the economy of the diesel engine is more down to it's increased compression ration over it's petrol equivalent, not energy density of the fuel.

[wolves_wanderer]

franv8 said:

I think you will find the economy of the diesel engine is more down to it's increased compression ration over it's petrol equivalent, not energy density of the fuel.

Possibly partly, but the fuel does also have a higher energy density

[leosayer]

and diesel is a lot heavier than petrol, so if you go by weight rather than volume, it isn't any more efficient.

[ubergreg]

Whatever the energy density, they're brilliant nowadays.

As a Canadian who's now living in the UK, I had never realized how effective oil-burners were - they simply don't market them in passenger vehicles in North America. I've driven quite a few diesels here; some of them are seriously quick, and all of them are more efficient than a petrol motor of the same size. Easy to see why they're so popular.

Don't understand why they're not more popular in across the pond. The tourque would really shift those bloated, heavy, thirsty Yank Tank SUVs to the ridiculous 60mph speed limit quickly, and with far less fuel. No soccer mommy would know the dynamic difference between petrol and diesel anyway.

>> Edited by ubergreg on Friday 28th January 15:19

[dieseljohn]

franv8 said:

I think you will find the economy of the diesel engine is more down to it's increased compression ration over it's petrol equivalent, not energy density of the fuel.

Also lack of throttling losses, wall wetting (direct injection) etc.

[franv8]

Apparently, I have made a small typographical error, I meant Compression ratio, not compression ration as originally posted.

I turn myself over to the grammar & spelling Police for sentencing...

By the way, I think petrol and diesel have similar relative densities at 0.8 against reference water. However it is true to say the engines themselves are considerably heavier. And they rattle a lot. And smoke. And don't rev. And if the latest VW diesels are anything to go by, the rather impressive power band exists between 3498 rpm and 3503 rpm.

[Pigeon]

They would rev more with Uncle Rudi's original air-blast injection system - gives much better fuel atomisation than a high-pressure-liquid injector, at the cost of a little complexity, but with reduced requirements for high-precision machining.

[v8thunder]

With the refinements in diesel technology leading to such smoothness and quietness nowadays, I can't see it being long before the luxury car makers like Rolls-Royce start using it to provide ultimate luxury and refinement with economy (and allowing us mere mortals a chance of running them!)

Cadillac needs to come back on board the idea of diesel power, and a Lexus diesel is long overdue.

[franv8]

I thought more of the issue with revving was the speed of burning - I know better atomisation would improve it - but would it get much better? It's not the spreading flame front seen through the 'homogenous' fuel-air mixture found in the petrol engine?

[Pigeon]

Oh yes, a more finely divided spray would ignite and burn much faster. If you look at the curve of cylinder pressure for a direct-injection diesel, not a lot happens for a significant while immediately after injection, while the relatively large droplets vaporise and mix with the air, then there's a sudden peak as it all goes off at once. With the improvement in atomisation and mixing from an air-blast injector, it'd start to burn as soon as it was injected.

Extremely finely divided combustible materials can be quite interesting when dispersed in air

[ridds]

Diesel technology is coming on in leaps and bounds these days.

Engine speeds are limited by maximum piston speeds due to their mass which is due to needed to the higher pressures seen in the combustion chamber (200bar). Blocks are becoming lighter with Compacted Graphite Iron (CGI) and Aluminium being used more and more. The easily recognisable knock associated with Diesels is gradually being iradicated with pre and post injection control systems that have now become even more flexible with electronic control systems and common rail fuel supply. Also higher rail pressures have allowed even more power.

What is worrying is the sensitivity of these parts now. Fuel pumps are very delicate and expensive items and are very prone to failure with the addition of conaminates. Water is one of the worst and will distroy a high pressure fuel pump in a few thousand miles. Italy has real problems with water in fuel. Kepp away from petrol stations that have just been restocked!!! This mixes teh water emulsion that collects in the tank into the fuel.

[speedy_thrills]

I’m sorry but have you knicked that picture from www.billzilla.org ?

[ATG]

speedy_thrills said:

I’m sorry but have you knicked that picture from www.billzilla.org ?

? Follow the link at the bottom of the story and scroll down.

[nel]

I was fascinated last year by the first ever entry of a V10 diesel car in Le Mans - 525 bhp but had to retire due clutch/transmission problems. See here
www.perkins.com/perkins/cda/articleDisplay/1,4094,7___32_____7_10022049,00.html

I still think they smell worse than petrol engines, but after a few years driving turbo-diesels overe here in France I'm beginning to appreciate the wave of torque feeling. Give it a few years and we'll have sportscars with V12 diesels!

[Lance Robinson]

The reason that Diesels are more economic that petrol engines is that they are always fully aspirated.

Specifically, air supply is never restricted, as it is in a petrol engine on less than full throttle). A diesel on part throttle still has the maximum amount of air coming in, but is given less fuel to burn.

This is more efficient as the engine is "free" on the air intake stroke. This is also the reason that diesels are far more economic in town where all driving is on part throttle.

[dieseljohn]

Pigeon said:

Oh yes, a more finely divided spray would ignite and burn much faster. If you look at the curve of cylinder pressure for a direct-injection diesel, not a lot happens for a significant while immediately after injection, while the relatively large droplets vaporise and mix with the air, then there's a sudden peak as it all goes off at once. With the improvement in atomisation and mixing from an air-blast injector, it'd start to burn as soon as it was injected. Extremely finely divided combustible materials can be quite interesting when dispersed in air

Pigeon - you know your stuff do you work in diesel combustion or is it a hobby (if so your insane )?

[dieseljohn]

ridds said:

The easily recognisable knock associated with Diesels is gradually being iradicated with pre and post injection control systems that have now become even more flexible with electronic control systems and common rail fuel supply. Also higher rail pressures have allowed even more power.

Pre-injection certainly does reduce diesel knock you should hear the difference when you turn it on and off on a test rig (maybe you have?). Even the best modern diesel still sound like tractors though IMHO.

[nel]

Pigeon said:

Oh yes, a more finely divided spray would ignite and burn much faster. If you look at the curve of cylinder pressure for a direct-injection diesel, not a lot happens for a significant while immediately after injection, while the relatively large droplets vaporise and mix with the air, then there's a sudden peak as it all goes off at once. With the improvement in atomisation and mixing from an air-blast injector, it'd start to burn as soon as it was injected. Extremely finely divided combustible materials can be quite interesting when dispersed in air

Not sure about this improvement in atomisation issue - the ignition of the charge in a diesel engine is initiated by the increasing pressure, so all the diesel injected goes bang at much the same time. Hence the distinctive knocking noise of a diesel engine compared to the progressive, flame-front burn away from the spark-plug in a petrol engine.

If you improve the atomisation of the injected diesel, you will improve the efficiency of the resultant explosion, but I do not believe that it will "start to burn" any sooner - the same critical pressure regime still has to be reached.

[speedy_thrills]

Would it be unfair to say that combustion in a compression engine is done in a more consistent manner than that of a spark ignition engine due to the ability to run at chemically “lean” (the amount of oxygen in the chamber exceeds that needed to combust the chemical fuel) charge mixture? (Assuming appropriate and even fuel dispersion throughout the cylinder)