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.
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