Fuel
brands and Grades
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y first
diesel car was reputed to be able to run on most fuels. Parafin, petrol, diesel, it would run. It may not run for long but it would
run. Lucy was the
same; she’d run on most fuels. But more
modern stuff is a lot fussier, in part because of the sophisticated injection
and fuel pump systems and in part because it’s all ECU
controlled.
For the longest time, different
grades of petrol have been available.
Years ago it was possible to buy two star, four star or five star
petrol. These days, a variety of manufacturers
sell a bewildering array of ordinary and premium fuels, many claiming to do
wonderful things for your engine.
Fuel Ratings
If you really want to know more
about your RONs, cetanes and octanes then this website isn’t the place for a
detailed technical discussion. I’m going
to gloss over the technical funky bits and merely provide an overview.
The most obvious difference
between unleaded, superunleaded and überunleaded is the RON rating of the fuel,
or octane. The higher the octane content
of the fuel, the harder the petrol resists igniting by being compressed. If an engine has a knock sensor, a higher RON
fuel will allow the timing to be changed and more power to be produced for a
given turn of the engine.
Ordinary unleaded is classified as
95 RON, superunleaded is 97 RON and www.dervman.com classifies anything of 98
RON or higher as an “überunleaded.” Shell Optimax was 98 RON, now replaced with Shell
V-Power, 99 RON. BP Ultimate 102,
available at select locations only, is 102 RON.
Tesco have also introduced a 99 RON überunleaded petrol.
Diesel’s cetane rating is the
opposite to petrol octane. The higher
the cetane, the easier the fuel is to ignite by compression, which makes for
smoother running, especially when the engine is cold. Using a higher cetane fuel can improve
combustion, meaning less soot or potentially increasing the power
available. However, a higher cetane
value does not mean a higher calorific energy content per unit of capacity; any
increase in power means that more diesel has been burnt. Diesel fuel supplied to the relevant British
Standards must have a cetane rating of 46, BP Ultimate has a minimum cetane
rating of 55 and Shell’s V-Power Diesel (at
the time of writing, only available at a limited number of stations) has
tested with a cetane value of 61.
Interestingly enough, BP Ultimate sold in mainland
Depending on who you believe, even
the latest modern high speed diesel engines are unable to benefit from a cetane
number over around 55; there is no benefit from using fuel with a higher cetane
number.
To further complicate matters,
diesel fuel has a cetane rating figure and a cetane index figure. The cetane index illustrates the “driveability”
of the engine. A low cetane index figure
means the driver will over-compensate with the accelerator pedal, in other
words the fuel isn’t very responsive.
Quite how the cetane index works is beyond me.
Are all fuels equal then?
The answer to this is, “it depends
on the car, the driver and the driving.”
Some drivers cannot tell if their car’s exhaust is hanging off let along
if the engine is running differently.
Other drivers swear that the latest überunleaded turns their otherwise
perfectly ordinary Vauxhall Corsa 1·2 into the
equivalent of the 1·8 SRi. The truth is
going to be somewhere between the two.
Kermy did feel different running
on Optimax.
The engine was smoother and quieter, but any reduction in fuel
consumption was not statistically significant.
Hoshi was definitely smoother
running on Optimax and used less of it.
When I sat down to first write
this article, results with Chef were inconclusive. Ordinary ASDA diesel is sootier than ordinary
Shell but just as economical. BP Cleaner
Diesel feels nicely responsive and there is materially less soot. Then I tried BP’s Ultimate Diesel, this
deserves its own mention.
The Überdiesels
At the time of writing, there are
three überdiesel fuels for same: BP’s Ultimate Diesel, Shell’s V-Power Diesel
and Total’s Excellium Diesel. These
three fuels share some common traits, the most important two are the
advertising hype and that they are a synthetic diesel fuel.
Without getting too bogged down in
the science, the majority of ordinary diesel fuel sold in the
Synthetic fuel, however, is
currently manufactured using Gas-to-Liquid technology. The liquid diesel fuel is much closer to
perfection, there are far fewer odd molecules.
This results in a cleaner burn.
It’s a gross oversimplification
and I don’t pretend to understand the process completely, other than if you dip
your fingers into one of the three überdiesels and sniff, it doesn’t smell like
diesel. This is because überdiesel
contains almost no sulphur, which is what gives ordinary diesel its distinctive
and delightful aroma.
Is Using An Überdiesel Fuel Worth
It?
Perhaps. My own evidence from the Saab TiD is currently inconclusive. On the one hand, it looks to be cost neutral. The fuel costs around 5% more to buy, but I
squeeze around 5% further from a tank. On
the other hand I don’t have encough information.
However, the 9-3 is smoother,
quieter and produces significantly less soot running on an überdiesel fuel.
BP Ultimate Diesel Case Study
You may have seen BP’s published
tests for their premium diesel fuel, Ultimate Diesel. The first test shows two identical Peugeot 106s accelerate and the car using BP Ultimate
diesel does extend a small lead.
The second test shows two Volkswagen
Bora TDIs being driven around a test track until they run out of diesel. The Bora on Ultimate Diesel goes
further. This test was repeated fifty
times.
Looking at the mathematics, the
Bora on BP Ultimate averaged an additional seventeen kilometres to a tank (best
result of an additional twenty eight kilometres). This is around eleven miles. This doesn’t sound too bad until we consider
what eleven miles is in the context of how far we would expect a Bora to manage
to a tank anyway.
The Bora has a fifty five litre
tank and the TDI 130 model (with cruise control, probably essential for this kind of test)
has a Datasheet touring range of six hundred and
sixty three miles. This is calculated
using the official fuel consumption figures modified to represent a touring
pace. It’ll do for now. Then we use the car running on BP Ultimate
Diesel, which covers another eleven miles.
So the car will travel for six hundred and seventy four miles. That’s a difference from 54·8 to the gallon
to 55·7 to the gallon or in percentage terms, 1·66%.
BP Ultimate Diesel costs a lot
more than an extra 1·66%, so from a cost perspective it’s impossible to justify
using this test result.
I have some thoughts on the
test. From the footage of the test track
it looked as though the cars were being driven at just fifty miles per
hour. Presumably at a constant fifty
miles per hour for a very long time until the tanks run dry. This is a long time at the wheel (a full day)
but in time terms, you only get another twelve minutes or so of driving before
you run out.
I can make up or lose those twelve
in the final two miles to and from home.
Also, if BP Ultimate Diesel’s
higher cetane rating improves the responses of the engine, surely a better test
would be to drive around a track where acceleration and braking is
required. The fuel with a higher cetane
index is more responsive, so the driver will end up using smaller accelerator
pedal movements.
Something like... oh... how about
the Nürburgring? See how many laps the
cars could manage when driving on the ‘Ring but with say a maximum speed of
seventy miles per hour applied?