tech torque
FRASER STRONACHDIESEL ENGINES ARE COMPLEX, BUT THEIR REAL
ADVANTAGE IS VERY SIMPLE.
20 http://www.4X4australia.com.au
Ethanol energy
P
ETROL may contain less
energy than diesel but both
contain a great deal more
energy than ethanol, which
offers a very poor energy return
on a volume or weight basis. In
fact, ethanol has one third less
energy by volume than petrol, or
put another way, petrol has 50
per cent more energy by volume
than ethanol.
Ethanol makes up 10 per cent
of E10 fuel, and ethanol’s lower
energy content is the reason
why your car won’t go as far on
a litre of E10 as it will on a litre
of straight petrol. E10’s lower
energy level erodes its lower cost
at the pump, sometimes making it
more expensive in the long run.
What’s not to be confused
here is that ethanol has a higher
octane rating than petrol, which
means it’s useful for engines
running high compression ratios
and/or forced induction such
as turbocharging. In other
words, it’s a good racing fuel
when you’re not concerned
about fuel economy.
If you’re a diesel driver who
thinks none of this is a concern,
you need to think again. We all
pay for ethanol via a federal
subsidy of around 38 cents per
litre, which makes it cheaper
for those people who buy it.
Effectively we are all paying for
ethanol, despite the fact that
it’s an energy-deficient fuel of
dubious environmental value.
Go figure.D
IESEL engines have come along
in leaps and bounds in recent
years thanks to a technology
revolution. That revolution is
based on electronic control of extreme-
pressure fuel injection systems and the
ever-more-sophisticated turbocharger,
which is now universally employed on
diesels, sometimes in multiples on any
one engine.
You probably think that’s why
a modern diesel-powered 4x4 will
typically use 20 or 30 per cent less
fuel than a petrol-powered equivalent,
but the fuel-efficiency advantage of a
diesel runs much deeper than that.
Most importantly, diesel fuel
contains more energy than petrol.
It’s a simple as that. The extra
energy in every litre of diesel will
take you further than the lesser
amount of energy in a litre of petrol.
The difference isn’t great but it’s
significant, and runs between six
and 16 per cent, the exact difference
being dependent on the specific
grade and blend of the diesel and
petrol in question.
Given the engine in your 4x4 merely
converts the potential energy in the
fuel into mechanical energy, the more
energy you put in the more energy
you get out. The trick is not to waste
too much along the way through heat,
noise, mechanical losses and the like.
The other inherent advantage of
diesel engines over petrol engines
is the mechanical efficiency of their
higher compression ratios. Modern
diesels run compression ratios that
are typically 50 per cent higher than
current petrol engines, and it wasn’t
long ago that diesels had compression
ratios twice as high as typical petrol
donks. The idea here is that by
squeezing more air/fuel mixture
into a small space you create a more
forceful explosion, which acts on
the piston to create more torque at
the crankshaft. Higher compressionratios generate greater pumping
losses than lower ratios, but the
extra energy used in compressing
the air/fuel mixture to a greater
extent creates a bigger bang in the
combustion chamber.
Why don’t petrol engines run high
compressions ratios like diesels? The
answer is again very simple and down
to an inherent virtue of diesel fuel.
Diesel is a far more stable fuel than
petrol and will stand much higher
temperatures before it will ignite. High
compression ratios create a lot of heat
and that’s what ignites the fuel in a
diesel engine.
If you try to compress a petrol/air
mixture to the compression ratios seen
in a diesel the result is an uncontrolled
explosion in the combustion chamber
called pre-ignition. Also called
‘knocking’, this uncontrolled explosion
can lead to catastrophic engine failure.
Given the advantage high
compression ratios bring, modern
petrol engines push the boundaries
of high compression via combustion-
chamber design, knock sensors and
high-octane petrol (among other
things), but could never run the sort of
ratios you see in a diesel.
Diesel compression ratios have
come down in recent years in order
to improve engine refinement, given
that lower compression means less
noise, harshness and vibration. There’s
also an environmental benefit, as
lower compression ratios lead to
reduced output of the various oxides of
nitrogen (collectively known as NOx), a
particularly nasty group of pollutants
and arguably the biggest stumbling
block for diesel engines in the future.
Greenies are big on this!
So next time you marvel at the
technology in a modern diesel engine
don’t forget about the inherent
benefits of diesel, a fuel that’s more
energy-rich yet at the same time less
volatile than petrol.