Thursday, 1 March 2007

The science and costs of diesel cars

Sorry for the delay in this post - but I have a very good reason....

For the second time in the last month, I was stranded in different city, this time Australia's capital, Canberra.

But it wasn't turbulence or aircraft delays that kept me there. It was car repairs, and these car repairs not only taught me a little about the science of diesel engines, but about how expensive diesel cars can be.

So this week's post is about the science and costs behind diesel cars.

The story begins with a lovely weekend away with the girl, staying at a great hotel, eating nice food and generally having my one decadent weekend for 2007. Having taken in Little Britain Live, I headed out of the gravel car park, not knowing at the time of the troubles that lay ahead.

To put this in context, I drive a VW Golf, and its a diesel car. Its a good car, and two reasons I quite like it are:
  • I do a lot of driving, long and short distance, and diesel cars are more fuel efficient than petrol cars,
  • Even though when you burn diesel it produces more greenhouse gases per litre than petrol, the fact that diesel engines are so efficient means that per kilometre you produce less greenhouse gas output than petrol.
Diesel itself is heavier than petrol as it is less refined. It weighs about 850 grams per litre, compared to around 720 grams per litre for petrol as it contains longer chained organic molecules (typically C14 vs C9). Essentially, these longer chained molecules release more energy when burned in the presence of oxygen, and this means that for the same volume of fuel, you get more energy and more carbon dioxide output (15% more in each case). Combined with the fact that diesel engines are intrinsically more efficient than petrol engines (~20-40%), you get lots of kilometres per litre (miles per gallon for our US readers) and less greenhouse gas output per litre.

Diesel is less refined that petrol. This means it is easier to isolate the heavier diesel from the crude oil that comes out of the ground than the lighter, more volatile petrol. One would think therefore that diesel should be cheaper, and it is in many parts of the world. Strangely, in Australia at the moment, diesel is more expensive than petrol. It seems to be because much less diesel is sold in Australia than petrol and so there are less competitive demands, and because it is not subject to the discount cycles that affect petrol - for a good explanation, see here.

Anyway, enough science and commerce and back to my story.

Driving through the gravel car park, a rock has flicked up, smashed the guard on the bottom of the car (see the gaping whole in the above picture), punctured something called the fuel cooler, which in turn bent the connection to the return fuel line, meaning that the plastic undercarriage of the car, the fuel cooler and the fuel line needed to be completely replaced, to the tune of $860 including labour costs.

Now this had me a little confused. What is this fuel cooler? What is a return line?

The first thing I discovered is that diesel coursing through the lines in a diesel car is under extremely high pressure, and so special plastic tubing must be used. This means that you can not simply cut out the part of the hose with the hole in it and clamp the two ends back together - you need to replace the whole thing. This is essentially why it cost so much - every damaged part needed to be replaced in its entirety (and imported from Germany I guess....)

There are a few differences between diesel and petrol cars. A petrol engine compresses the fuel at a ratio of 8:1 to 12:1 - this ratio is the ratio between the volume of a combustion chamber when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more energy that can be obtained, as there is more fuel and oxygen crammed into the same space on ignition. Diesel engines use a compression ratio of 14:1 to 25:1. This higher compression ratio leads to increased efficiency.

Unlike a petrol engine, a spark plug is not needed to ignite the fuel, as this compression raises the temperature above the flash point of the mixture - when a gas is compressed, its temperature rises. Diesel is directly injected into the combustion chamber, where compressed air is injected and the highly compressed mixture ignites. This is unlike a petrol engine where fuel and air are mixed before entering the chamber, and then the mixture is ignited by a spark plug. Petrol can not be compressed to the same extent as diesel as the compression would raise the temperature above the flash point well before this ratio was reached. This is why diesel engines are intrinsically more efficient than petrol engines.

I believe the return line returns diesel that is not used in the combustion back to the fuel tank. It would seem that this diesel is at extremely high temperature and pressure, and so needs to be cooled. This is where the fuel cooler tube comes in. A punctured fuel cooler and broken return line meant that about 20 litres of diesel leaked into car park, but luckily there was enough diesel floating around in the lines to get me to the nearest service station.

A couple of days later, after many phone calls to the service people and to VW, I got my car back. Unfortunately, this does not fall under warranty as it was an "accident" - fair enough in theory as no parts failed (one could argue the plastic on the bottom of the car really should be able to sustain the impact of rocks in a car park when travelling at low speed however...), but very annoying in practise as it was completely accidental with no one at fault. It also meant I didn't qualify for free towing - so I'm avoiding paying that remarkably large bill too! ($200 for towing across 2 suburbs??)

Anyway, enough whining, its a good story and I must be zen about it - I am lucky to have a car at all, and just recently have heard some horror stories about car damage. It taught me some nice science and gave me another opportunity to chat to my insurance company! I am sure I have got some of the science of diesel cars wrong, so if you can correct anything I have said in this post, please do. I would like to know more myself! Let me know if anything similar has happened to you.

This will be out on the podcast - mp3 here.
For some more info on diesel engines, check out How Stuff Works.


  1. Well, about getting some of the science wrong, I don't aggree when you said "The higher the compression ratio, the more energy that can be obtained, as there is more fuel and oxygen crammed into the same space on ignition" - this is because the amount of fuel you can put into the cylinder depends only on how much air is in the cylinder (in the case of petrol in a 1:14 ratio, 1 part petrol to 14 parts air), and since the air is sucked into the cylinder and THEN compressed, then the amount of air (and hence amount of fuel) in the cylinder is independent of the compression ratio. To put it another way, if you have a 1.4-litre engine then it will suck in a little less than 1.6 litres of air (and a little petrol, make 1.6 litres of stuff total), regardless of the compresion ratio. By compressing the fuel more, you simply have the same mass of air in a smaller space, which means you'll only be able to inject the same amount of fuel into it to get complete combustion.

    However, you were right about higher compression leading to greater efficiency - the proof of that comes from thermodynamic theory which I will not bore you with here :)

    BTW, adding a turbo to compress the incoming air DOES increase power, because a turbo will allow the engine to suck in more air in the same volume. For example, if the engine above were to be attached to a turbo which doubles the pressure of the air, then it's like your 1.4 litre engine sucked in 2.8 litres (equivalent) of atmospheric air, so you can inject twice the fuel and it would still burn completely, and hence you'd get twice the power, minus the power needed to run the turbo. Notice that practically all diesel engines have a turbo...

  2. sorry, I meant to say "if you have a 1.4 litre of engine, it will suck in almost 1.4 litres of air" (not 1.6 litres)

  3. Thanks Luke - interesting stuff. Seems it's more of a density thing - I think this is what I meant to write, but perhaps worded it poorly. Thanks for reading and commenting!

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