4×4 Engine Petrol vs. Diesel
It is a well proven fact that diesel engines fitted to off-road vehicles are more fuel/energy efficient than comparable petrol engines, however, there are many factors that influence the actual differences. Apart from the design differences between the two types of 4×4 engine, the single most important issue is the fuels themselves, and although they have a common origin, crude oil, their chemistries are vastly different, giving different combustion characteristics and thus different power delivery at different engine RPM’s.
The rate at which 4×4 vehicles consume fuel is of critical importance, especially during an overland expedition in Africa, where fuel supplies can be erratic and the quality dubious. But given the inherent energy inefficiency of internal combustion engines in general, the issue becomes one of extracting the most energy from the least amount of fuel, so let us compare apples with apples:
• Based purely on the calorific value of fuel, the petrol engine should win the debate outright; however, let us compare apples with apples:
• Calorific value of petrol: 45.8 MJ/kg (mega joules/ kilogram)
• Calorific value of diesel: 45.5 MJ/kg (mega joules /kilogram)
That is one side of the equation; the other is how much energy is contained in any given amount of fuel:
• Energy in one litre of diesel: approx. 36.9 MJ/litre (mega joules/ litre)
• Energy in one litre of petrol: approx. 33.7 MJ/litre (mega joules/litre)
But there is another side to this issue; the relative densities of the two fuels:
• Density of petrol: approx. 0.72 kg/l
• Density of diesel: approx. 0.85 kg/l
Thus by being around 15% denser, diesel contains more energy per litre than petrol, which means that when combusted,
• diesel delivers about: 38.6 MJ/litre (mega joules /litre), whereas
• petrol delivers only delivers about: 34.9 MJ/litre (mega joules /litre),
from which can be seen that diesel delivers more usable energy per given volume than petrol. Taking all these factors in account, it will be seen that diesels are on average around 20% more energy efficient than petrol engines, which are at best 25-30% efficient, whereas diesels run at efficiencies of at least 40% and more. In short, this means that your off road vehicle gets more movement from a litre of diesel than from a litre of petrol.
However, from a 4×4 perspective, there are other, more pertinent issues when it comes to choosing between petrol and diesel powered off-road vehicles. Both engines have advantages and disadvantages, some of which from a 4×4 perspective are:
• It does not need only diesel to run:
In a pinch, it can be run on cooking oil, olive oil, peanut oil, palm oil, strained used engine oil, and of course biodiesel. Except for the latter, which can be used on a permanent basis provided it conforms to ASTM standards, vegetable oils should be used for short periods only since prolonged use can lead to filters and injectors clogging up. The use of paraffin mixed in any ratio with anything else should be avoided at all costs because paraffin has no lubricating properties and it will as a result destroy an engine and/or injection equipment in very short order.
• High torque at low RPM’s:
Almost all diesel engines develop around 90% of their power (KW) at just above idling RPM’s. However, they need around 2500 RPM to develop all their torque, which is around half of the RPM’s needed by a comparable petrol engine to reach maximum torque. All this means that diesel engines outlast petrol engines several times over while using less fuel.
• Lower fuel consumption:
Off road diesels use on average around 40% less fuel than comparable petrol engines.
• Diesels run cooler:
Ambient temperatures in parts of Africa reach 45-50 deg C almost on a daily basis, which does not do the cooling systems on petrol engines any good. In addition, the exhaust temperatures of diesels are up to 200 deg C cooler than those on petrol engines, which from the 4×4 perspective, makes good sense when driving through long grass. Many an off-road vehicle has burnt out because of grass collecting around the hot exhaust systems of petrol engines. Although 4×4 diesels are not totally immune to the problem of grass collecting around hot exhausts, the risk of that grass catching fire is much higher with a petrol engine.
• Fewer electrical components:
There are not many repair facilities in the hinterland of Africa that are equipped with diagnostic computers to trace electrical failures. From a 4×4 perspective, the fewer electrical components that could potentially fail, the happier you will be, especially ignition circuits that could get wet and fail in the middle of a tricky river crossing.
4×4 diesel engines are significantly heavier that comparable petrol engines, adding to the overall weight of an off-road vehicle.
Diesel vehicles of all types generally have a significant price penalty because of the higher cost of producing diesel engines.
• Short service intervals:
Although service intervals have increased somewhat in recent years, an overland expedition through Africa of say, 20 000 km’s, will require at least 3 oil changes. The quality of diesel fuel in much of Africa can be dubious at times, which can have severe negative effects on the durability of engine oils. However, the use of fully synthetic oils can mitigate these effects but they are very expensive, and if you do not take your own, you will be very lucky indeed to find some in, for instance, Timbuktu.
While petrol engines are not bad, and can be used successfully in off-road vehicles just about anywhere in the world, they have no significant advantages over a diesel engine. In fact, their distinct disadvantages compared to a diesel engine make them impractical to use for anything but short 4×4 excursions where if they fail, or something goes wrong, you will be within easy reach of a repair facility.
• Fuel consumption:
Any given petrol powered off-road vehicle has on average only around 60% of the range of a diesel. Long range tanks take up space and add weight, and even if you managed to fit a long range tank, you might still need to carry additional fuel just to reach the next refueling point.
• Electrical issues:
The engine and fuel management systems on modern petrol powered off-road vehicles are complex to the point of ridiculousness. Even if you do have a “limp” mode on your ECU, the Trans-continental highway crossing the Congolese rain forest is not the place to see if it works.
• High RPM’s:
Having to use 5000 RPM’s or more to have enough torque is fine for short periods, however, having to use that rev range for even just 40 or 50 km’s at a stretch, using 4×4 mode in low range especially in Africa, and in 45+ deg C heat, may cause you to run out of fuel just before your petrol engine expires due to overheating.
From all of the above it should be clear that although petrol engines have a place in the 4×4 universe, diesel engines are without doubt the better choice. However, the last word on engine choices has not been spoken; no matter what you may know or not know about the relative energy densities of automotive fuels, the single most important thing all off-road drivers have strong opinions about is the performance (or lack thereof) of their favourite off-road vehicles.
Even though the two terms, Rated KW and Torque are not the same thing, many 4×4 enthusiasts and even mechanics use them interchangeably, so what are they and how do they differ?
Power vs. Torque
Power, whether measured in Nm, HP, BHP, or any other accepted term, is nothing more than the rate, or speed at which a certain amount of work can be done in a specified time. The most commonly used definition of horsepower is: 33,000 lb.ft. per minute. Look at it this way: to work at the rate of one horsepower, you would need to lift 33 000 pounds one foot high over a period of one minute. One horsepower is equal to 745.699921 Watts.
Torque, and not power, is what moves a 4×4 through thick sand or up a steep hill. To do this, a sufficient twisting or rotational force referred to as “torque”, needs to be applied to the wheels, and the amount of this force is what determines whether your off-road vehicle reaches the top of the hill or not.
In the Imperial system, torque is measured in pounds at the end of 1foot long lever or arm, and in the metric system as Nm at the end of an arm or lever that is 1 metre long, thus; a 100 pound force applied to a 1 foot long arm will read as 100 lbs/ft of torque and 100 kilograms of force applied to a 1 metre long arm will read as 100Nm of torque. The equivalent forces are:
• 1 lb/ft equals 1.356 Nm
• 1 Nm equals 0.737 lb/ft
But how does torque work? Simple: Imagine you are trying to loosen a really tight wheel nut on your 4×4; by using a 1 metre long wheel spanner or wrench, the force applied to the end of it will cause the nut to turn, with the amount of force, or work applied to the end of the spanner being the torque required to loosen the wheel nut. By shortening the spanner, double the work will be required to result in the same torque reading and by doubling the length of the spanner, the same torque can be obtained by applying only half of the power to the end of it.
Which is exactly what happens in a 4×4 diesel engine: because of the bigger off-set between the centre of the bearing journal and the crankshaft centre line when compared to a petrol engine, less power is required to turn the crankshaft, exactly like the lower amount of power required to loosen the wheel nut with a longer than 1 metre wheel spanner. This is why diesels can have lower KW or HP ratings than comparable off-road petrol engines; because of their longer piston strokes, diesels develop more torque, or rotational force, with less input power. Clear?
Which 4×4 engine then?
This is not an easy question to answer: there are many factors that count both for and against both types of 4×4 engine and as with football teams; each has its die-hard supporters. If you already own one or the other, there is little point in replacing it just for the sake of change.
However, if you are thinking of buying a 4×4 for the first time, do not rush into the first good deal you find. Issues of cost and higher maintenance of diesels could for instance, preclude you from getting all the recovery gear you must have to make your trip safe and trouble free, especially if you are on a budget. Nor should personal preferences be allowed to cloud your best judgement; off-road driving is a serious business, and potentially dangerous, so your choice should rather be determined by where you are most likely to use the vehicle, how often, your budget and most importantly, your skill level- for instance, do not buy a diesel with a manual transmission just for the sake of buying a diesel because it uses less fuel, if you are not comfortable with manual transmissions.
Best of all, make an informed decision; if you are unsure about what is the best, petrol or diesel, join an off-road club. Collectively, the members possess vast experience and knowledge on all things 4×4 and paying close attention to the clubs’ collective past experience, is the surest way to make the right decision.
For more on the 4×4 engine see the below links: