Petrol vs. Diesel
Much off-roading is done at low speeds (as
slow as possible, as fast as neccessary), using
a lot of power. This requires an engine that produces high amounts of
torque at low revs (aka grunt). Diesel engines, by their very nature,
are low-revving, high-torque engines, although modern, purpose-built
petrol engines have similar characteristics. Entire rain forests have
been destroyed documenting the virtues of one or the other, and I
present some arguments in favour of each.
As mentioned above, diesel engines are well-suited to off-roading
because of their low-revving, high-torque nature. However, diesel
engines are much less responsive than petrol engines. Other
considerations include availability of diesel, and their suitability
Availability of fuel in your country must be the first consideration.
Diesel is available almost everywhere, although, because of its high
sulfur content in widespread areas (such as Africa) require a very high
degree of maintenance and therefore cost. Diesel injection systems are
complex and therefore difficult to understand, and expensive to fix or
replace, especially in remote areas.
Lastly, because diesel engines induce combustion through compression,
they are well-suited for wading through water.
Petrol engines, on the other hand, produce similar torque, but at much
higher revs. This becomes especially aparent when descending steep slopes,
as it will be seen that diesel-powered vehicles descend much slower than
their petrol counterparts. Bear in mind, however, that a large engine in
a small car (such as the 4.0l straight six found in the Jeep Wrangler)
provides an excellent power-to-weight ratio. Even though maximum torque
is produced at higher revs, it will still descend a slope slower than a
Defender TDi or Pajero 2800 Turbodiesel. Petrol engines are also much
more responsive than diesel engines. "Feathering" the throttle in a petrol
engine, coupled with effective clutch control, allows for greater driving
control and better driving flexibility than diesel-engined vehicles.
Of course, most mechanics world-wide understand petrol engines, and
frequency as well as cost of maintenance and repairs are substantially
lower than on diesel engines. On the down-side, however, many modern
petrol engines use electronic injection systems which can be as complex
as diesel counterparts. Further, most modern petrol-powered four wheel
drives run on unleaded fuel, which is not always readily available in
remote countries such as Morocco, Mozambique and Zambia.
The last point worth mentioning is that while practically feasable and
still cost-effective, it is much more difficult and expensive to
water-proof a petrol engine for wading than it is a diesel engine.
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