----- Original Message -----
From: "jonathon" <jemery@execpc.com>
To: "Military Vehicles List" <mil-veh@skylee.com>
Sent: Sunday, July 02, 2000 10:10 PM
Subject: Re: [MV] Out of curiosity...
>
> > Out of curiosity, how is the Hercules Multifuel engine different from
any
> >other diesel engine?
> > I know what would happen if one attempted to burn gasoline in a
[standard]
> >diesel. It would be spectacular, if short lived...
> > Since acquiring my deuce, everybody and their brother asks me how it is
> >modified for multifuel use, and I haven't a clue.
> > Any takers?
>
Well, I'll have a go. . . . . . . .
> In general a compression ignition engine (Diesel) will burn lighter fuels
as
> you increase the compression ratio. I believe the LD's run something like
> 22 or 23 to 1 compression ratio. A normal garden variety Diesel is in the
> vacinity of 16 to 1. If you've ever gotten gas in your Diesel you know
that
> you start loosing power and you actually stop detonating the fuel, it
sounds
> like the engine is missing. The injection pump on an MF engine has extra
> goodies to increase fuel delivery based on fuel viscosity as energy
content
> drops as the fuel gets lighter (shorter carbon chains or smaller number of
> carbon atoms in the molecule) as well as some lubrication issues.
>
As a rule of thumb the 12 - 15:1 CR diesels are direct injection and the
18 - 22:1 CR types are indirect. Direct is just that, you have an injector
in the combustion space much like where you'd find a spark plug in a gas
(petrol) engine. These are easy starters often without any form of start
heating but are slow revvers and usually have a very characteristic diesel
knock, modern design has improved the situation greatly though. To generate
cylinder swirl for good combustion the larger types may have deflector vanes
on the backs of the inlet valves and pistons with toroidal crown
indentations to encourage the air to swirl violently, four hole injectors
are often used also.
Indirect injection has the injector in a separate air-cell typically the
size of a table tennis ball and connected to the cylinder via a small port,
say 1/4" square. The piston comes within a few thou of kissing the head on
TDC and the air cell port is offset to the cell thus generating a very
violent tornado of air in the cell. These are high revving engines with
good combustion properties but the cooling surfaces presented to the air
require some form of starting aid like heaters in the cells to secure a
start and need the higher CRs to run.
Nothing burns except vapour, those things that do not vaporise at normal
temperatures have to be heat or mechanically vaporised before burning, the
better the vaporisation the better the burn. Its easy for gas as its flash
point (temperature where sufficient vapour is given off for combustion) is
so low, just spray it into an air stream using a carburettor, you do not
have to be really good at breaking the liquid fuel up as it wants to do this
anyway. Diesel oil or other fuels don't do this because the flash point is
very high so a mechanical means has to be sought, injectors work at
extremely high pressure with miniscule apertures, rather than spray they
generate a fog, the ultimate aim is to get a few molecules of diesel fuel
surrounded by air for combustion. The diesel cycle realises more energy
from a lower energy fuel (conversion efficiency) because of the increased
compression pressures but this requires the cylinder charge to be
non-volatile until required necessitating a complex and critical injection
method.
Using gas in a diesel engine without some compensation will have it explode
in short order mainly owing to the greater calorific content so a multifuel
injection pump has to "read" what its handling and make appropriate
compensations, the biggest hassle is pump lubrication. Diesel oil and other
fuels, except kerosene perhaps, are actual oils and have lubricating
properties and it is the fuel alone that normally lubes the pump, neat gas
must be desperate for the critical and highly loaded internal bearing
surfaces hence the addition of some oil.
Some surprisingly innocuous substances will "diesel", Dr Rudolf Diesel had
originally intended for his CI engine to burn the substantial quantities of
scrap coal dust available at the time and air-injected this does work,
however, bore life owing to the ash formed is measured in minutes. Flour
will diesel quite happily ! Mills have to take mining precautions because
of the highly explosive nature of flour dust when individual particles have
a jacket of air, puff half a teaspoonful out of a tube and hold your zippo
to the small cloud formed, beware. As has been mentioned the truly huge
ship diesels of typically 20,000 HP or more do burn very heavy, thick
sludge-like stuff and in the case of crude oil tankers the actual cargo oil.
In the case of Kuwait crude this is kept heated in the ships tanks to be
pumpable and heated to around 100C for use in the engine, at room temps the
stuff is like candle wax.
> The reverse of this is can you burn Diesel in a gas engine. The answer is
> yes, but again compression ratio comes into play. Old spark ignition
> engines routinely ran kerosene but they had compression ratios as low as
> 3'ish to 1. Any high than that and the heavy fuels will detonate before
you
> want them to, consider running low octane gas in a high compression gas
> engine, knock knock. The theory of combustion in compression and spark
> ignition engines are radically different.
>
Perhaps some confusion here, the old kerosene engines usually used what I
know as Tractor Vaporising Oil (TVO) which is very like AVTUR or domestic
heating oil. They are essentially gas engines with seriously oversized
manifold hot-spots, after starting on regular gas and getting the exhaust
manifold up to temperature it would heat the inlet manifold sufficiently to
allow the poorly carburettor vaporised TVO, after tank switch over, to be
heat vaporised sufficiently for spark ignition, the hot inlet charge though
makes for poor volumetric efficiency and low specific power output.
Invariably you find a carb float bowl drain tap on these old engines to
allow the carb TVO to be drained before selecting the fuel switch to gas for
the next cold start.
Knocking, the big difference between CI and spark ignited engines is that
the former take on a charge of non volatile air, there is nothing in the
cylinder to go bang unlike spark ignited engines, combustion starts when the
fuel is injected. They have a characteristic idle knock since the fuel
often burns, blows itself out and the remaining injection quantity then
detonates later, the effect goes away with higher revs and increased
injection ie., "throttle" open. There is a huge distinction in all internal
combustion engines between fuel burning or explosion; and detonation where
the charge goes bang all at once, there is but a few milliseconds between
the two.
Richard
Southampton - England
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