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Engine & fuel engineering FAQ

Engine & fuel engineering

Diesel and Gaseous Fuels, DualFueling by franzh
Posted: 14 Aug 05

Dual fuel systems may be used with turbocharged Diesel applications.  These engines have been historically been used for:
* irrigation pumps;
* farm tractors;
* over the road tractor-trailer rigs;
* utility and delivery vehicles;
* personally owned medium to heavy-duty trucks used for either everyday driving or the occasional hauling.

Before we get into the system, I would like to dispel some myths about dual fueling.
First, you can do considerable damage to an engine if it is dual fueled to excess.  In the past, it was not uncommon to see engines with welded or melted pistons, rings, cylinder heads, gaskets, valves, and cylinders!  Hmm, done a few myself!  What has happened is that the vehicle owner decided that "if a little bit of fuel is good, a lot is better!"  If opening a little screw 1/8 turn, 1 full turn is better!  

It is notoriously simple to add lots of a gaseous fuel to a diesel engine and produce copious amounts of torque and horsepower.  Dr. Diesel stated in his original manuscript the concept of adding "Erdgas" (Earth Gas, or Methane, or more commonly known, Natural Gas) to the incoming air stream.  He found that adding (by volume) approximately 35% Methane to the air stream improved engine power by almost 50%.  (Remember that these engines were produced in the early 1900's and diesel engine design was in its infancy.)

There are simple rules of thermodynamics, this is the most basic:
It takes heat to produce power in any internal combustion engine, and it takes fuel and air to produce heat.  The more heat produced, the more power is generated, up the point of destruction caused by the inability of the engine to absorb or reject the heat properly.  Since diesel engines operate in the "excess air mode" adding more fuel simply makes the combustion process hotter.

To dispel other rumors:
Propane is NOT used as a catalyst to "more completely burn the fuel".  A catalyst changes the chemical structure of the fuel, period.  Modern diesel engines burn almost all of the fuel (in the 99% plus range, see below).

Also, fuel is NOT wasted out of the exhaust.  It is the engines mechanical responsibility to take advantage of the burned fuel.  Taking an exhaust analysis from a running engine shows the amount of unburned fuel to be in the region of less than .001%, pretty tiny!  

Taking a scale of the amount of heat that is produced during combustion, and where it is used shows some interesting things:
In a diesel engine (not spark ignited), about 35% of the combustion energy actually goes to the rear wheels.  The rest is consumed in the following ways, within reason, and with some flexibility due to engine and vehicle differences.  This is further broken down in three separate areas:
Actual brake engine power
Thermal losses from radiation
Thermal losses from the exhaust
From these numbers, we then extrapolate these figures:
12% is radiated from the engine radiator;
10%  from thermal losses through the block through heat radiation;
45% is lost through the exhaust waste heat (a little less if the engine is turbocharged);
About 5% of the energy is consumed by the process of combustion, the physical conversion of chemicals into gasses;
About 10% is lost due to engine motoring friction losses, piston drag, camshaft bearings, lifters, crank drag, oil pump, water pump, valve and rocker arm friction, etc.
Unless a magical means of eliminating these values is discovered, you will NEVER see an engine produce much more than about 40% efficiency, from BTU's of raw heat from burning the fuel, to usable power at the wheels.

The use of LPG or Natural Gas as a fumigation fuel does several things:
1.  It displaces some of the air in the combustion chamber, requiring the use of a turbo to regain some of the air-mass density.  I am not saying that you cannot fumigate a non-turbo or normally aspirated diesel engine, only that you will not obtain the full benefit if it is not turbocharged.

2.  A gaseous fuel (LPG in this case) has different burn characteristics.  Diesel has approximately 155,000 BTU's per gallon, LPG only 91,500 BTU's, per liquid gallon, or only 62% of the raw heat energy of diesel..  Diesel combusts by compression, and has a critical compression ratio beginning around 12:1, depending on the combustion chamber temperature (the hotter, the lower the compression needs to be to ignite the fuel).  LPG also has a critical compression ratio, somewhere around 12:1 too, but its spike or combustion pressure rise time is MUCH quicker than diesel.  That's why you hear the distinctive rattle and gray smoke when too much LPG is applied into a diesel air intake, (the gray smoke is unburned fuel, and can be VERY combustible).

3.  It is possible to pull another 20%+ power from a gen II 5.9 24 valve Cummins, but be VERY careful of turbo temperatures.  Install a thermocouple BEFORE the turbo.  Exhaust temps should NOT exceed 900 to 1000 degs, if it does, lean the fuel mixture, YES, LEAN the fuel mixture.  One website shows a 4-wheel drive Ford smoking all 4 tires from a standing start, pulling a trailer!  Impressive?

4.  As a diesel engine powers up under full load, the diesel fuel begins to displace the air.  LPG vapor is then admitted into the air stream, which further displaces air.  The turbo is required to dump great amounts of air back to regain the lost power.  Remember that its the air that makes the engine power, not dumping vast amounts of fuel in. (That's why the turbo is there, to pump air, not fuel!)

The goal is to develop a substitution fuel engine, or an engine that burns an alternative fuel other than that fuel which the engine was originally designed for.  In this case, the engine which was designed for diesel fuel use, we now substitute approximately 20% LPG (Propane).  I am well aware of competitive vehicle kits that offer driver adjustable fuel systems that promise (at least verbally) horsepower gains of up to 40%, and gain as much as 15% fuel mileage, ostensibly by allowing the propane to act as a "catalyst"!

Again, if you look at the rule of thermodynamics, to put it simply, "you can't get something for nothing".

Consider:  You must burn approximately 38% more propane than diesel to achieve the same power level.  This is why it is not that simple, just switching one fuel for another.

One other concept. . . .Diesel engines operate unthrottled, the principal of a diesel engine is that the engine speed and torque level is controlled by the amount of fuel injected or consumed during the intake stroke.  Diesel fuel has a very wide air-to fuel ratio, making it ideal for the diesel Compression Ignition (CI) principal.  Diesel has a combustibility limit of from 5 to 35% air to fuel by weight.  Any more than that and great clouds of brown and black smoke show up (unburned fuel).  Any less than that and the engine will not power up.  LPG has very precise lean and rich limits of 2.1 to 9.6% by weight.  As it reaches the last 1% of that region, either lean or rich, the engine will lose the ability to run effectively.

Propane and Natural Gas have a narrow air to fuel ratio (by comparison to diesel), therefore in order to maintain an efficient combustion, the amount of air must be reduced to meet the ideal air to fuel ratio of the gaseous fuel (and gasoline too, for that matter).  This ideal ratio is called the "Stoichiometric Ratio".  The process of mixing the air with enough fuel to fully consume both during combustion describes the technical process of an efficient combustion.  

Another thing to consider.  Modern vehicles have to pass emission tests in order to be sold in the US, and in many other parts of the world.  If you alter any OEM design criteria, and it deteriorates from the certified emission production of that vehicle, you have voided the OEM warranty, and can incur tremendous fines and penalties from the EPA.  Not one website offering these fumigation kits mention anything about EPA testing or certification.
If you exceed the original vehicle manufacturers engine horsepower and torque design specifications, you void any warranty and will probably incur engine damage, turbocharger damage, cracked exhaust manifolds, etc.  I attended an automotive technician training conference in early 2002 where the attendees were told that when a diesel powered vehicle was admitted into their repair facility, they were to examine the vehicle carefully for signs of a "recently removed" fumigation system, including:
Tank mountings
Engine induction system modifications
Wiring splices
Cooling system splices
. . .and if there were such signs, the warranty would be voided!  One such website plainly states that their system is "easily removable if the vehicle must be taken to a dealership for warranty repair"!

Also, there are several methods of fumigating a diesel engine.  Lets explore:
1)  Installing a venturi just before the turbo.  This method is very popular, and relatively accurate since it provides fuel directly proportional to the amount of air entering the engine (and also relative to the amount of turbo boost).  Since the diesel engine is unthrottled, there is the same amount of air entering the engine (per crankshaft revolution) at idle as it is at full throttle, minus the additional air provided by the turbo.  The venturi provides a vacuum signal, again proportional to engine load and RPM, to the pressure regulator / reducer / converter, which then sends fuel, based on the amount of vacuum demand.  Simple but relatively accurate.  This is currently the most popular method.

2)  Installing a spud pipe directly in the turbo inlet.  This is the absolute in simplicity, but very inaccurate in fuel metering.  The vacuum signal may be dependent on the presence and quality of the air filter element(!) and can change drastically.

3)  Installing a pressure regulator that meters propane vapor at about 3 psi ABOVE turbo pressure (the pressure regulator must be balanced against turbo pressure), which then bleeds fuel into the intake manifold, often directly at the intake ports.  This system is very fast reacting, and can dump copious amounts of fuel, and a lot of experimentation must be done to obtain the right amount of fuel mixtures at loading.
User installed orifices are generally used to restrict the fuel flow.   It is prone to over-fueling, but does work.

4)  Electronic fuel injection.  This is absolutely the best method of fuel metering, but is expensive, and each engine must be "mapped" for its fuel supply vs. engine load.  Also, various sensors must be installed, such as a TPS sensor, air temp sensor, exhaust temp sensor, engine rpm, manifold vacuum/pressure or a mass air flow sensor.  Many modern fuel management systems can also present a pre-set fuel map that just uses pressure, rpm, and temperature, but the more input the management has, the better the outcome.

At this time, there are just a few successful companies using this method, but are currently only developing systems with Natural Gas, but research has been done using LPG.

I was recently given some of the evidence presented during a successful lawsuit.  Disclosure laws prevent me from exact details, (I don't know the exact details anyway) but the amount requested during the suit was USD $15,000 (the actual amount of damage), plus the amount paid to rent a replacement vehicle (about USD $4,000).  The court awarded more than three times the amount requested in punitive damages because the defendant attempted to obscure the evidence by introducing other evidence not relevant to the case.  The defendant included the installer who was the "manufacturer" of the kit.

The engine had approximately 1,000 miles on the conversion, and approximately 15,000 miles on the vehicle (2000 model).

The information provided is my own, any reference to existing documentation is coincidental.

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