Re: the sexiest aircraft

On Wed, 18 Jan 2006 14:03:41 -0500, "Daniel A. Mitchell"
<danmitch@xxxxxxxxxxx> wrote:

>Diesel tanks were also far less prone to 'brewing up' (catching fire)
>when hit, and were less likely to roast their own crews.

This is a well known "factoid" about diesel tanks, and the M4 Sherman in
particular. However the evidence is that there's no difference in
flamability between WW2 diesel and petrol tanks. Tanks of this period
were rear-engined, had fuel tanks in the same compartment, and had
rudimentary ammunition stowage. Where they burned, it was because
ammunition propellant had been hit, not fuel. Most tanks for most of
WW2 also carried solid shot AP ammunition, not HE, and so there was no
explosive on board. A propellant fire will destroy the tank, but
generally by a powerful fire rather than an explosion. Although tanks
certainly burned, it was ammunition causing it, not fuel.

The Allies' safest tanks against this hazard were the late model M4A3
Shermans that switched to wet stowage of ammunition (incidentally,petrol
fuelled).

> However, their
>performance suffered compared to contemporary gasoline engined tanks.

In what way ? The US' diesel tank experiments were as a result of
fitting the Guiberson radial diesel to the M3 light tank as a substitute
for the Wright Continental W-670 radial. This was a perfectly
satisfactory engine. It only caused problems when the Guiberson was
then fitted to the far heavier M3 medium tank (1 1/2 times the weight of
the light tanks, or the earlier M2 medium). This was an underpowered
vehicle with the Wright engine too - the petrol version generally had
the more powerful R-975.

The later diesels (the GM 6-71 twin diesel and the Caterpillar RD-1820)
were comparable in power to the petrol engines, but had generally better
torque characteristics and much better fuel consumption figures. The
Caterpillar incidentally was only used in the M4A6 and just 75 of them
were produced.

Neither the US nor the UK entered the war with a decent tank engine. The
UK was using WW1 Liberty aero engines in the cruiser tanks (powerful,
but infamously unreliable) and various twinned commercial truck or bus
engines in the infantry tanks (very successful, but only suitable for
smaller tanks). The US was dependent on radial aero engines (production
needed for aircraft) and crash programs to twin commercial diesels. Not
until mid-war did they really develop a dedicated tank engine, the
superb British Meteor (a land-based Merlin) and the horrible Ford GAA
V8. As always with US tank production, a pragmatic decision to
facilitate production led to choosing a technically poor design, then
producing it in overwhelming quantities.

In WW2 there was no performance penalty to using diesel engines in
tanks. This was due to a simple factor - the poor quality of available
petrol and its minimal octane rating. In comparison, wartime diesel
suffered far less of a performance drop. The petrol Meteor could only
achieve 1/2 the output of the very similar Merlin, due mainly to the
relative qualities of "pool" petrol and Avgas.

>In later years, as Diesels with better power to weight ratios were
>developed, the U.S. army switched to Diesel engines in their tanks. This
>occurred after the Korean war, about midway through the run of M-48
>"Patton" tanks.

This was the third mark of the M48. The M48A1 went to the M48A2
basically by supercharging the first petrol engine. Power was almost
unchanged, but fuel consumption dropped to 2/3rd. The M48A3 switched to
a turbo diesel engine (also used in the M60) to gain another 30%
improvement.

>Nowadays almost all tanks are Diesel powered, with the notable exception
>of the turbine powered M1 "Abrams" tanks.

Since the1960s it has been a bit of a misnomer to refer to "diesel"
engines in tanks. The switch to compression ignition was driven by
efficiency, but the fashion was also for multi-fueling (which really
depends on using CI). There are CI and SI engines, but the fuel is a
matter of availability.

Both UK and US engine fleets are now free of petrol engines and are
purely diesel. Only motorbikes, chainsaws and some US generators still
require petrol - the UK replaced its generators a few years ago and both
are still searching for a workable diesel motorbike.

The Abrams gas turbine was also developed as a multi-fuel engine, which
commonly burns diesel. Jet A-1 is perhaps the "ideal" fuel, but has
little difference from DERV (vehicle diesel) - equally a Challenger will
happily run on jet fuel. Early Abrams models could burn petrol too
(badly - this is hard for a gas turbine) but I understand that this
facility is no longer used.


--
'Ph'nglui mglw'nafh Cthulhu Evesham wagn'nagl fhtagn'
.

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