Diesel + CNG Dual-Fuel System

Our dual-fuel system for medium and heavy commercial vehicles will consistently displace up to 70% of diesel fuel with natural gas. In the dual-fuel configuration, diesel pilot injection ignites the CNG, making externally-supplied ignition unnecessary. In addition, dual-fuel systems offer greater independence in areas where CNG infrastructure is incomplete, since they are fully capable of running on diesel alone.

 System Layout
 Dual-System Layout


CNG System Components

 

CNG Type 4 tank

 

 

Type 4 UltraLight Carbon Fiber CNG Tank

    • NGV2-07 & ISO 11439 Certified
    • CNG Capacity: Up to 46.5 Diesel Gallon Equivalent
    • Service Pressure @70oF: 3,600psig
    • Size: Up to 25”x90”
    • Weight: Up to 280lbs
    • Life: 20 years

NG Pressure Regulator

 

 

High Pressure Regulator

    • Temperature Range: -22oF to 221oF
    • Outlet Pressure: 123psig
    • Inlet Pressure: 3600psig
    • Gas Flow Rate: Up to 140lbs/hr
    • Coil Voltage: 12-24VDC
    • Sensor Signal Output: 0-5V

 Natural Gas injector

 

 

High Pressure Injectors

    • Excellent response characteristics
    • Robust and compact design at 16mm body diameter
    • High flow rate up to 320 lbs/hr
    • Can be used for bi-fuel or dual-fuel applications
    • High impedance
    • Integrated pressure and temperature monitoring

 Gas Mixer

 

 

Gas Mixer

    • Naturally aspirated and turbocharged engines
    • High homogeneity at very low resistance
    • Robust design and construction

 Cng Control Unit

 

 

CNG Engine Control Unit

    • Controls all CNG injectors, pressure regulator,
      tank shutoff valves, and pressure sensors
    • Communicates with the stock ECU to optimizez
      CNG & diesel operation
    • Can be used for bi-fuel or dual-fuel applications

Natural Gas Safety

Any motor vehicle fuel can be dangerous if handled improperly. Fuels contain energy which must be released by burning. Gasoline is a potentially dangerous fuel, but, over time, we have learned to use it safely. The same is true of compressed natural gas (CNG). Natural gas safely generates our electricity, heats our homes and cooks our meals. But, like gasoline, CNG must be understood and respected to be used safely as a fuel.

CNG has safety advantages compared to gasoline and diesel: it is non-toxic, and has no potential for ground or water contamination in the event of a fuel release. An odorant is added to provide a distinctive and intentionally disagreeable smell which is easy to recognize. The odor is detectable at one-fifth of the gas’ lower flammability limit.

CNG, unlike gasoline, dissipates into the atmosphere in the event of an accident. Gasoline pools on the ground creating a fire hazard. The fuel storage cylinders used in NGVs are much thicker and stronger than gasoline fuel tanks. The cylinders are constructed of metal, composite materials, or a combination of the two. NGV fuel systems are "sealed," which prevents any spills or losses to evaporation. Even if a leak were to occur in a NGV fuel system, the natural gas would dissipate up into the air because it is lighter than air.

Natural gas has a high ignition temperature of about 1,200 degrees Fahrenheit, compared with only about 600 degrees Fahrenheit for gasoline. Natural gas also has a narrow range of flammability, which means that in concentrations below about 5 percent and above about 15 percent when mixed with air, natural gas will not burn. The high ignition temperature and limited flammability range make accidental ignition or combustion of natural gas less likely.

Time has proven NGVs to be safe in actual operation. Based on a survey of 8,331 natural gas utility, school, municipal and business fleet vehicles (NGVs) that traveled 178.3 million miles:The NGV fleet vehicle injury rate was 37% lower than the gasoline fleet vehicle rate. There were no fatalities compared with 1.28 deaths per 100 million miles for gasoline fleet vehicles.The fleet of 8,331 NGVs was involved in seven fire incidents, only one of which was directly attributable to failure of the natural gas fuel system.

Source: NGVAmerica

Understand Natural Gas Engine Emissions and Control Technologies



 

Emissions

The primary criteria pollutants from natural gas-fired reciprocating engines are oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOC). The formation of nitrogen oxides is exponentially related to combustion temperature in the engine cylinder. The other pollutants, CO and VOC species, are primarily the result of incomplete combustion. Particulate matter (PM) emissions include trace amounts of metals, non-combustible inorganic material, and condensible, semi-volatile organics which result from volatized lubricating oil, engine wear, or from products of incomplete combustion. Sulfur oxides are very low since sulfur compounds are removed from natural gas at processing plants. However, trace amounts of sulfur containing odorant are added to natural gas at city gates prior to distribution for the purpose of leak detection.

It should be emphasized that the actual emissions may vary considerably from the published emission factors due to variations in the engine operating conditions. This variation is due to engines operating at different conditions, including air-to-fuel ratio, ignition timing, torque, speed, ambient temperature, humidity, and other factors. It is not unusual to test emissions from two identical engines in the same plant, operated by the same personnel, using the same fuel, and have the test results show significantly different emissions. This variability in the test data is evidenced in the high relative standard deviation reported in the data set.

 


 

Nitrogen Oxides:

Nitrogen oxides are formed through three fundamentally different mechanisms. The principal mechanism of NOx formation with gas-fired engines is thermal NOx. The thermal NOx mechanism occurs through the thermal dissociation and subsequent reaction of nitrogen (N2) and oxygen (O2) molecules in the combustion air. Most NOx formed through the thermal NOx mechanism occurs in high temperature regions in the cylinder where combustion air has mixed sufficiently with the fuel to produce the peak temperature fuel/air interface. The second mechanism, called prompt NOx, occurs through early reactions of nitrogen molecules in the combustion air and hydrocarbon radicals from the fuel. Prompt NOx reactions occur within the flame and are usually negligible compared to the level of NOx formed through the thermal NOx mechanism. The third mechanism, fuel NOx, stems from the evolution and reaction of fuel-bound nitrogen compounds with oxygen. Natural gas has negligible chemically bound fuel nitrogen (although some molecular nitrogen is present).

Essentially all NOx formed in natural gas-fired reciprocating engines occurs through the thermal NOx mechanism. The formation of NOx through the prompt NOx mechanism may be significant only under highly controlled situations in rich-burn engines when the thermal NOx mechanism is suppressed. The rate of NOx formation through the thermal NOx mechanism is highly dependent upon the stoichiometric ratio, combustion temperature, and residence time at the combustion temperature. Maximum NOx formation occurs through the thermal NOx mechanism near the stoichiometric air-to-fuel mixture ratio since combustion temperatures are greatest at this air-to-fuel ratio.

 


 

Carbon Monoxide and Volatile Organic Compounds:

CO and VOC emissions are both products of incomplete combustion. CO results when there is insufficient residence time at high temperature to complete the final step in hydrocarbon oxidation. In reciprocating engines, CO emissions may indicate early quenching of combustion gases on cylinder walls or valve surfaces. The oxidation of CO to carbon dioxide (CO2) is a slow reaction compared to most hydrocarbon oxidation reactions.

The pollutants commonly classified as VOC can encompass a wide spectrum of volatile organic compounds that are photoreactive in the atmosphere. VOC occur when some of the gas remains unburned or is only partially burned during the combustion process. With natural gas, some organics are carryover, unreacted, trace constituents of the gas, while others may be pyrolysis products of the heavier hydrocarbon constituents. Partially burned hydrocarbons result from poor air-to-fuel mixing prior to, or during combustion, or incorrect air-to-fuel ratios in the cylinder during combustion due to maladjustment of the engine fuel system. Also, low cylinder temperature may yield partially burned hydrocarbons due to excessive cooling through the walls, or early cooling of the gases by expansion of the combustion volume caused by piston motion before combustion is completed.

 


 

Particulate Matter:

PM emissions result from carryover of noncombustible trace constituents in the fuel and lubricating oil and from products of incomplete combustion. Emission of PM from natural gas-fired reciprocating engines are generally minimal and comprise fine filterable and condensible PM. Increased PM emissions may result from poor air-to-fuel mixing or maintenance problems.

 


 

Carbon Dioxide, Methane, and Nitrous Oxide:

Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are referred to as greenhouse gases. Such gases are largely transparent to incoming solar radiation; however, they absorb infrared radiation re-emitted by the Earth.

 


 

Control Technologies:

Three generic control techniques have been developed for reciprocating engines: parametric controls (timing and operating at a leaner air-to-fuel ratio); combustion modifications such as advanced engine design for new sources or major modification to existing sources (clean-burn cylinder head designs and prestratified charge combustion for rich-burn engines); and postcombustion catalytic controls installed on the engine exhaust system. Post-combustion catalytic technologies include selective catalytic reduction (SCR) for lean-burn engines, nonselective catalytic reduction (NSCR) for rich-burn engines, and CO oxidation catalysts for lean-burn engines.

Control Techniques for 4-Cycle Rich-burn Engines

 


 

Nonselective Catalytic Reduction (NSCR):

This technique uses the residual hydrocarbons and CO in the rich-burn engine exhaust as a reducing agent for NOx. In an NSCR, hydrocarbons and CO are oxidized by O2 and NOx. The excess hydrocarbons, CO, and NOx pass over a catalyst (usually a noble metal such as platinum, rhodium, or palladium) that oxidizes the excess hydrocarbons and CO to H2O and CO2, while reducing NOx to N2. NOx reduction efficiencies are usually greater than 90 percent, while CO reduction efficiencies are approximately 90 percent.

The NSCR technique is effectively limited to engines with normal exhaust oxygen levels of 4 percent or less. This includes 4-stroke rich-burn naturally aspirated engines and some 4-stroke rich burn turbocharged engines. Engines operating with NSCR require tight air-to-fuel control to maintain high reduction effectiveness without high hydrocarbon emissions. To achieve effective NOx reduction performance, the engine may need to be run with a richer fuel adjustment than normal. This exhaust excess oxygen level would probably be closer to 1 percent. Lean-burn engines could not be retrofitted with NSCR control because of the reduced exhaust temperatures.

 


 

Prestratified Charge:

Prestratified charge combustion is a retrofit system that is limited to 4-stroke carbureted natural gas engines. In this system, controlled amounts of air are introduced into the intake manifold in a specified sequence and quantity to create a fuel-rich and fuel-lean zone. This stratification provides both a fuel-rich ignition zone and rapid flame cooling in the fuel-lean zone, resulting in reduced formation of NOx. A prestratified charge kit generally contains new intake manifolds, air hoses, filters, control valves, and a control system.

 


 

Control Techniques for Lean-burn Reciprocating Engines:

Selective Catalytic Reduction (SCR)

Selective catalytic reduction is a post-combustion technology that has been shown to be effective in reducing NOx in exhaust from lean-burn engines. An SCR system consists of an ammonia storage, feed, and injection system, and a catalyst and catalyst housing. Selective catalytic reduction systems selectively reduce NOx emissions by injecting ammonia (either in the form of liquid anhydrous ammonia or aqueous ammonium hydroxide) into the exhaust gas stream upstream of the catalyst. Nitrogen oxides, NH3, and O2 react on the surface of the catalyst to form N2 and H2O. For the SCR system to operate properly, the exhaust gas must be within a particular temperature range (typically between 450 and 850oF). The temperature range is dictated by the catalyst (typically made from noble metals, base metal oxides such as vanadium and titanium, and zeolite-based material). Exhaust gas temperatures greater than the upper limit (850oF) will pass the NOx and ammonia unreacted through the catalyst. Ammonia emissions, called NH3 slip, are a key consideration when specifying a SCR system. SCR is most suitable for lean-burn engines operated at constant loads, and can achieve efficiencies as high as 90 percent. For engines which typically operate at variable loads, such as engines on gas transmission pipelines, an SCR system may not function effectively, causing either periods of ammonia slip or insufficient ammonia to gain the reductions needed.

Catalytic Oxidation

Catalytic oxidation is a post-combustion technology that has been applied, in limited cases, to oxidize CO in engine exhaust, typically from lean-burn engines. As previously mentioned, lean-burn technologies may cause increased CO emissions. The application of catalytic oxidation has been shown to be effective in reducing CO emissions from lean-burn engines. In a catalytic oxidation system, CO passes over a catalyst, usually a noble metal, which oxidizes the CO to CO2 at efficiencies of approximately 70 percent for 2SLB engines and 90 percent for 4SLB engines.