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Diesel Dual Fuel

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Diesel Dual Fuel Systems

One of the best applications for dual fuel is for stationary engines which are typically used for power generation, pumping, and running compressors. In this case, the engines are designed to consume natural gas with a small, pilot injection of diesel fuel to ignite this fuel mixture. Natural gas is used in this application because it has an extremely high resistance to self ignition from the high compression-ratios used in diesel engines. The critical compression ratio (CCR) of natural gas is about 15:1, depending upon the amount of methane in the fuel. Like a conventional single-fuel diesel engine, there is no throttle valve and load control is done by adjusting the fuel mixture. These engines are among the most efficient in terms of kW produced per Joules consumed (hp produced per BTU consumed) because of the thermal efficiency of the diesel cycle.

In the automotive world, there are kits available to convert your diesel truck engine to dual fuel operation. However, the important difference between automotive engines and stationary engines is that automotive engines generally rely on propane as the gaseous fuel and this is simply because propane has a higher energy density than natural gas. When comparing propane with natural gas, propane has a lower resistance to self-ignition. Propane's Anti-Knock Index or Octane Number is much lower than natural gas' (104 compared with 120+) which limits the static compression ratio to around 12:1. A typical diesel engine has much higher compression ratio than this so a propane-fueled diesel engine will knock severely with sufficient amounts propane.

In an automotive diesel dual fuel application, propane provides increased energy storage over natural gas because approximately 4 to 5 times more propane can be stored in the same physical space as compressed natural gas. However, CNG is often much cheaper than propane and many vehicles like school buses and pickup trucks often have ample room to place a sufficiently large CNG cylinder onboard.

Although dual fuel systems can be described as fumigation systems or injection systems, both commonly supply a gaseous fuel vapor upstream (one system, downstream) of the turbocharger. Injection systems simply use injectors (rather than a mixing device) to provide a more precisely controlled amount of fuel to the engine. No system (that we're aware of) offers port injection, where a precise amount of fuel is supplied to individual cylinders. Instead, all systems provide a premixed fuel-air mixture to the engine and diesel fuel is injected into each cylinder to ignite the homogeneous gaseous fuel mixture.


Diesel Dual Fuel Conversions

Franz Hofmann has written excellent articles for the Eng-Tips forum FAQ. We highly recommend that you read them if you are considering a dual-fuel conversion for your diesel engine. He has participated in a great deal of research with dual fuel fumigation, both natural gas and propane (see Expert advise on diesel propane fumigation and Propane enrichment for diesels) and is very much against driver-controlled LPG metering. For this reason, we also recommend that you stay away from any driver-adjustable system!

There is great deal of misinformation on the internet and, if your engine is damaged by a dual fuel system, you will be on your own. In reading through the FAQ of one dual fuel system, we came across the following statement:

Should your vehicle require a trip to the dealer for routine maintenance or repairs, the XXXX™ is easily removable and labeled for ease in reinstalling.

This statement clearly acknowledges that their system will result in warranty issues with the manufacturer . Now that these systems are in existence, your dealer will be looking for signs of a dual fuel system installation when you bring it in for service on an engine failure.

Another bit of misinformation is the idea that LPG is a catalyst to improve the combustion diesel fuel. By definition, a catalyst is a substance that causes or speeds up a chemical reaction by the presence of the catalytic substance that in itself remains unchanged. In a diesel engine, for practical purposes, all fuel is already consumed. With a dual-fuel system, both the LPG (or other gaseous fuel) and the diesel fuel will be consumed so there is no way that LPG could possibly be considered a catalyst.

Another dual fuel system web site states that:

Propane when injected into a Diesel Engine enhances combustion by evening out the normally violent explosion that commonly occurs in Diesel Combustion. This evening effect provides a more complete burn, a cleaner burn, a cooler burn and a less violent burn resulting in lower EGT's, more power, more torque, fewer emissions and the system pays for it self with huge increases in fuel mileage!

It is interesting that they would describe Diesel combustion as a "normally violent explosion". Diesel fuel is commonly injected directly into the combustion chamber and relies on the heat of compression to ignite the fuel. The fuel droplets must first vaporize before they can ignite. Long before the fuel mixture approaches stoichiometric, the engine will start producing smoke. Diesel combustion is quite complex and it is extremely simplistic to say that diesels are inefficient and have incomplete combustion because of the smoke produced as engine load approaches maximum. That smoke, although sometimes quite visible, actually represents a very small amount of unburned fuel. Very little smoke, if any, is produced at low loads. However, diesel particulate emissions are definitely hazardous to your health and are present even at idle. See CARB School Bus Study. There is also evidence that soot has a definite detrimental effect on climate (see National Geographic - Melt Zone).


The Otto Cycle, which is considered to be a constant volume process, is one in which the fuel mixture reacts so quickly that the piston moves very little during the combustion process. Theoretically, the Diesel cycle is considered to be a constant pressure cycle which means that, as fuel is being sprayed into the combustion chamber, the fuel combusts about as fast as it is sprayed-in. which results in a much slower rise in cylinder pressure. In practice, the fuel injection duration is very short and the combustion is quite fast and is more closely approximated theoretically by the thermodynamic Dual Cycle. The characteristic diesel clatter (violent explosion?) is due to the very fast rise in pressure during combustion due to ignition delay (the time between injection and ignition).

For more information about diesel dual fuel combustion, see the following paper:

Because Diesels do not have throttle valves and rely on the amount of fuel injected into the combustion chamber to control the power produced, Diesels often operate at very lean fuel mixtures. Adding a gaseous fuel to a Diesel's intake charge allows the engine to make more power because it reacts with the excess oxygen already in the engine. However, Stewart reports that the gaseous fuel combustion is dependent upon its equivalence ratio, the characteristics of the pilot injection, and load. Due to its high Octane Rating, LPG & CNG allow the Diesel engine to perform more like an Otto cycle engine under some conditions, thereby slightly increasing its thermodynamic efficiency. The co-combustion of a gaseous fuel with diesel fuel generally reduces diesel particulate (soot) emissions, which reduces Regen Cycle time thereby further improving fuel economy.

Although a higher thermodynamic efficiency tends to reduce exhaust temperatures, additional fuel can only increase combustion temperatures because the system introduces more energy to and reduces the diluting air from the combustion chamber. In order to produce more power, more fuel must be consumed which must result in higher combustion chamber temperatures. Because the heat losses from the engine have not decreased significantly, there is no way for there to be a huge increase in fuel economy. The gain in fuel efficiency is due to the additional amount of work (see Thermodynamic Cycle) possible by the increase in thermodynamic efficiency. Keep in mind that, although you will be buying less diesel fuel, you will be buying more LPG/CNG, and LPG contains less energy per unit of volume than diesel. Only if your local propane or CNG price is significantly lower than diesel will you be substantially reducing your driving costs. Typically, natural gas obtained at a public CNG station run by a natural gas utility will much less expensive (on a BTU basis) than any other fuel.

Obviously, an engine will produce more power when it consumes more energy and this requires putting more fuel into a cylinder per cycle. In practical terms, diesel engines are designed to operate with excess air and adding more fuel will ultimately make combustion hotter, thereby making more power from the resulting higher temperatures and pressures. It is safe to increase the power output of a diesel engine by adding small amounts of propane or natural gas to the air stream. Too much gaseous fuel will cause it to self-destruct. Generally, low loads allow proportionally more propane/CNG to be used relative to diesel fuel. As loads increase, the proportion must be reduced to prevent the onset of highly destructive detonation. CNG's higher CCR allow more fuel substitution than LPG.

Because a gaseous fuel will displace some of the combustion air (as it would in a gasoline engine as well), a turbocharger is the best way to obtain the full benefits from the addition of a gaseous fuel. Ideally, the amount of propane/CNG added to the fuel mixture must be carefully controlled by an electronic fuel metering system, which takes into account many engine variables.


For many applications, Diesel Dual Fuel systems can make a lot of sense and can offer a very good payback. For LPG applications, we offer the Technocarb EcoDiesel System and the ACME Red Rooster Diesel Economizer. Both the Technocarb EcoDiesel System and the ACME Red Rooster Diesel Economizer are available in diesel-CNG and diesel-LNG versions as well (EcoDiesel CNG & Red Rooster CNG). One of the EcoDiesel System's standard features is an “Idle Emissions” mode, which is designed for school buses and other vehicles that spend a great deal of time idling.

 

 

Other Diesel-Propane Commercial Systems

Other Diesel-CNG/LNG Commercial Systems

For those people looking for a budget diesel dual fuel system, see DIY Diesel Dual Fuel. For more information about natural gas for heavy duty trucks, see LNG Conversions.