Diesel vs Petrol
by Mark Peacock

One of the many choices you have to make when buying a 4WDrive is whether to have a diesel or petrol engine. There is a lot of folklore and myth that a "real" 4WDrive HAS to be a diesel, but there are some good arguments for a petrol engine. Now, before any diehard 4WDriver complains that I must be biased if I even dare to consider a petrol 4WD, let it be known that I drive an 80 series 4.2 diesel cruiser.

Although many diehards may disagree, the main thing to remember is that both fuel types have advantages and disadvantages and there is no definitive answer on which is "the best". At the end of this article is a summary on all the advantages and disadvantages about petrol and diesel engines, so if you want to cut through the "waffle" click here to go to the summary.

Purchase Price
The first thing one notices when looking at buying a diesel power plant is that it typically commands a $1,000 - $3,000 price premium (and that's before a turbo is added to the options list). This is due to the higher manufacturing costs of a diesel engine because of its design. A diesel engine is built stronger than a petrol as it typically has a compression ratio of 20:1 compared to the petrol's 9:1. The stronger construction also accounts for the higher weight of the diesel engine, generally 20% higher.

Performance
Perhaps the most common (and only partially correct) conception of the diesel engine is that they are slow compared to a petrol engine (note, I am always comparing similar capacity engines unless otherwise stated). But when you add a turbo to a diesel, then the equation changes. A turbo diesel has the equivalent top end power of a petrol while still having the low down torque of a diesel and the same good fuel economy. While this may sound like the perfect solution, the problem is the extra cost. One of the advantages of the diesel is it's good fuel economy but the extra cost of the turbo would more than pay for the increased fuel cost.

For 4WDriving, one of the greatest advantages of the diesel engine is its low down torque. When in first low, it's very hard to stall and will simply crawl over rugged terrain. A petrol vehicle will often require use of the throttle to stop it stalling and this results in jerky progress. This advantage is far more pronounced on rocky ground and hill climbs and fades away for sand and beach driving.

Engine Braking
Another distinct advantage of the diesel engine is its engine braking. The design of the diesel engine means that it sucks in air unrestricted and the amount of diesel fuel injected determines how much power the engine makes. A petrol engine requires a constant fuel/air ratio so the amount of air drawn in by the engine is regulated by an inlet buterfly in ratio to the fuel being used. What this means is that at idle, a diesel engine still draws in a large amount of air (RPM x engine capacity) while a petrol has the butterfly almost closed and there is very little air being drawn in.

When using  engine compression for downhill braking and  factoring in the compression ratio of 20:1 for a diesel and only 9:1 for a petrol, the diesel engine offers significantly greater resistance to an increase in engine RPM.  The diesel engine offers significant resistance as large quantities of air are still being drawn in through the unrestricted air intake and being compressed to 20:1. The petrol engine has the air intake butterfly closed during downhill braking and so only minimal quantities of air is being drawn in and then, its only being compressed to 9:1. This is why a petrol engine tends to "run away" compared to a diesel.

Maintenance
Maintenance on a diesel is generally less complex, but more frequent. A diesel should have an oil change every 5,000kms and an oil filter every 10,000kms. A typical petrol will require an oil change at 10,000kms with the filter at 20,000kms, and sparkplugs are usually required along with the filter change. Major service items like diesel injectors, fuel pump etc typically require servicing every 100,000kms. Other maintenance items like transmission oil, air filters etc... are independent of engine type and have the same service interval.

If you do your own lube and oil changes, a diesel will generally work out cheaper but more frequent. If you pay for your minor services, the higher frequency of the diesel services will generally cost more than the less frequent but more costly petrol services. Remember though, this is a generalisation and will differ for different makes and models.

The design parameters of a diesel engine result in it having a longer life between rebuilds. Generally, a diesel will run up some 350,000 kms before requiring a rebuild (provided it has been well maintained) whereas a petrol may require a rebuild at 250,000 kms. While a diesel may take longer between rebuilds, a petrol is cheaper to overhaul so in the long run it works out about even.

Fuel Economy
This is where a diesel engine really performs. It will average between 20 to 50% better than its petrol counterpart. However, it will not have the same power so in some situations a diesel has to be worked a lot harder tp perform and this takes its toll on the fuel economy. Overall in every day driving, you would expect at least a 25% improvement.

Other advantages are that its better fuel economy help improve its range (for a given tank capacity). This is especially helpful in remote off-road driving. Also, diesel is a "safer" fuel due to its lower volatility. An uncapped petrol container will produce a vapour trail that can be ignited up to 20m from the  fuel container. This has proven particularly hazardous when a vehicle has been refuelled from a jerry can and the vapour trail drifts towards the campfire.

Exhaust Temperature
Once again due to the diesel's design, its exhaust temperature is around 200 degrees Celsius lower than a petrol engine. While in on-road driving this isn't much of a problem, off-road it has lead to major catastrophes. A typical problem in outback Aussie 4WDriving is the dry spinifex type grass that grows in the middle of tracks. This gets caught up in all areas, especially exhausts, and has been the cause of many  fires. Many a 4WD has been lost this way. A diesel is not immune to this problem, but a petrol is at a far higher risk due to its hotter exhaust temperature.

Water Rating
4WDrives and water crossings tend to go hand in hand and its no surprise that a diesel shines again. Provided that water does not get into the air intake (or fuel tank), a diesel will operate underwater. When a petrol engine is subjected to this torture, it will invariably fail due to the electrics shorting out. Typically the sparkplug leads will short out, or the distributor will fail.

However, while a petrol tends to stall before any damage is done, a diesel will suffer major internal damage if water gets into the air intake. With its 20:1 compression ratio, only a very small amount of water will cause dramatic failure of a diesel  engine. A petrol is not so prone to this as it does not draw in as much air (hence less water) and only has a 9:1 compression. Thus any water that is drawn in isn't subjected to as high a compression.

In practise, a diesel engine comes through most water crossings easily where a petrol engine often splutters its way through with intermittent shorting of the electrics. A well prepared petrol engine will minimise this, but not totally eliminate it.

Summary
There are many factors in the choice of diesel versus petrol with the most important being what you plan to do with your 4WD. A diesel engine is generally less powerful, more fuel efficient, has lower maintenance costs but with higher rebuild costs. As for its suitability for off-road driving, few would argue over the distinct advantages a diesel offers, but not all are convinced that it outweighs its poorer on-road performance. I guess it boils down to just how much off-road driving you plan to do.

           (c) 4WD Encounter 1998

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 These pages were last updated on 28 May 2004.
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