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Understanding HPDI Natural Gas Fuel Systems

By Paul Pate, Training Manager, NGVi    

There are two different types of natural gas engines and systems for the heavy-duty natural gas market: spark ignited and compression ignited.  Spark-ignited systems are by far the most numerous, and account for more than 90% of the market. There is a limited number of compression-ignited systems available. These systems, which only exist as dual-fuel systems, come in two types: substitution systems and high-pressure direct injection (HPDI) systems.

Substitution systems simply add natural gas to the diesel fuel, “substituting” a percentage of the diesel with natural gas. These trucks will run either on natural gas mixed with diesel or pure diesel, but will not run on natural gas only. The substitution percentage of diesel with natural gas ranges between 0-65%, dependent upon load and operating conditions. This type of system accounts for the majority of dual-fuel diesel/natural gas engines.

HPDI systems are vastly different in that they only use diesel fuel as a “liquid spark plug,” and only about 5% of the total volume of fuel combusted is diesel. An HPDI system will not effectively operate on diesel only, and requires natural gas as the primary fuel. In fact, there is a run timer within the engine control module (ECM) of an HPDI system that only allows 600 seconds of engine operation if the fuel pressure is too low for natural gas injection. This low-pressure cut off is set at approximately 2,500 psi. The reason the low-pressure cut off is set at such a high pressure is that the natural gas must be directly injected into the engine cylinder, which is pressurized as the piston moves from bottom dead center (BDC) toward top dead center (TDC) during the compression stroke. There must be enough gas pressure to overcome the cylinder pressure so that the natural gas will actually flow into the cylinder.

HPDI systems are limited to liquefied natural gas (LNG) vehicles. Their fuel conditions are very different from conventional LNG systems. Previous HPDI systems utilized unsaturated LNG fuel. Unsaturated means that the fuel has not been warmed as has conventional saturated LNG fuel. Saturated fuel is approximately -200°F and 100 psi, while unsaturated fuel is typically about -230°F and 30 psi. The newest HDPI system scheduled to enter the market is designed to run on either saturated or unsaturated fuel. For the purpose of this article, we will focus only on those systems running the unsaturated fuel.

Other than the storage temperature of the fuel, the internal operation of the LNG tank, and related components, the HPDI engine components operate the same.

Inside the unsaturated LNG tank is a high-pressure cryogenic pump. The role of this pump is to convert the low-pressure cold LNG into high-pressure gaseous state natural gas. The power for the cryogenic pump is taken from an auxiliary hydraulic pump attached to the back of the truck engine’s power steering pump, with hydraulic hoses that carry this pressure run from the power steering pump to the LNG tank. This pump is a dual-action pump and produces pressure on both the “extend” and the “retract” stroke.

In the cryogenic pump, the low-pressure and low-temperature cryogenic liquid travels through the piston stroke and is heated as it is compressed, beginning the vaporization change of state. An internal vaporizer uses heated engine coolant as its heat source then completes the warming process and fully vaporizes the LNG into high-pressure CNG. This gaseous fuel travels to the high-pressure accumulator, which smooths out the pulses from the cryogenic pump. The pressure in this vessel is as high as 4,300 psi. There is a safety relief device on these cylinders rated at 5,000 psi. Since these are high-pressure storage cylinders, HPDI fuel systems will require inspection per FMVSS 304 standards, ensuring that they meet NFPA 52 codes for high-pressure safety.

High pressure from the accumulator then flows to the fuel conditioning module (FCM), mounted on the engine. The FCM is where the fuel is directed to the individual injectors, located within the combustion chamber. These injectors are unique, in that they have two injection ports: one for natural gas, and the other for diesel. The diesel port fires first and injects a small amount of diesel fuel which ignites due to the heat of compression. Milliseconds later the high-pressure natural gas is injected into the cylinder, and the already combusting diesel ignites the gas. In order for the diesel side of the system to work, the diesel fuel pressure must be high, so there are transfer and pressure pumps as well.

The diesel side of the fuel system includes a fuel tank, a diesel transfer pump, and a high-pressure diesel pump, delivering fuel to the FCM. Because diesel is combusted, there will be exhaust aftertreatment including DEF, DPF and SCR, although the size of these items can be reduced due to the very small amount of diesel actually burned.

These HPDI systems are intended for high torque and horsepower applications. The manufacturer of the fuel system’s components is Westport, and the original system was applied to the Cummins ISX 15L, although production on that system has ceased. Currently, Westport is working with Volvo to produce this system in their D13 liter engine, and it is expected to hit the market in 2015. Obviously, servicing these systems will require additional training, because they have many unique components.

So you might ask yourself why you would be interested in a more complicated system that will require additional cost, components from both diesel and spark ignited engine types, and a new technology for technicians to learn. According to Frank Bio, Director of Sales Development, Specialty Vehicles and Alternative Fuels with Volvo Truck, it’s all about maintaining the ability to provide higher horsepower and torque than is available with current spark-ignited CNG engines. Additional advantages Frank lists for this system include decent engine braking due to the high (18:1) compression of the diesel engine, getting the same fuel economy as a diesel powered vehicle and less generated engine heat.

Regardless of your initial impression of the viability of this technology, remember that spark ignited CNG engines were new to the heavy-duty market in the not too distant past, and now they are becoming mainstream. The great news for all of us is that technological advances are being made in our industry, and they are what will make us better, stronger, faster and more reliable, whether in their current form, or in some iteration of the technology as it improves over time.