Technology

Natural Gas Engines

Currently two different types of engines are used in air compressor applications: industrial and automotive-derivative.

Industrial engines are typically designed to operate for longer lifetimes than automotive-derivative. These engines operate at speeds up to 1,800 RPM. The time between major overhauls or rebuilds on this type of engine is in excess of 20,000 hours of operation. Industrial engines are designed so that all the parts that experience wear can be replaced. These major overhauls allow the life of an industrial engine to be extended “indefinitely.” However, industrial engines are more expensive than automotive-derivative.

Automotive-derivative engines are rated for higher speed (more than 3,000 RPM) operation and are modified versions of those used in automobiles and other mobile applications. These engines are less costly than the industrial grade engines, and therefore can decrease the capital cost of a project. Other benefits of these engines are that they are somewhat smaller and lighter in weight. The cost and availability of parts is generally better than industrial engines. However, the life of these engines is significantly shorter. Automotive-derivative engines are generally replaced at the end of their useful life, and are not rebuilt like the industrial engines. Therefore, the reduced installed cost of this type of engine is countered by the increased operating costs of replacing the engine at regular intervals. Also to be factored in is the downtime to overhaul an industrial engine (2 weeks) versus replacing an auto-derivative engine (2 days).

Analysis to assist in deciding between engine types should consider the following:

  • Life cycle cost comparison (first cost; operating, repair, and maintenance costs; and replacement cost)
  • Duty-cycle (operating hours)
  • Customer reliability requirements

Overview

Gas engine technologies are available for a variety of applications that require a prime mover. A prime mover simply converts one form of energy (gas or electricity) to mechanical work (turning a shaft) for use in a process. Thus, a prime mover can be an electric motor or a gas engine.

Gas engines perform the same function as an electric motor, they provide shaft work for process applications. Shaft work includes turning a shaft to operate a pump, a compressor, a grinder, a crusher or a generator. In many applications an electric motor can be replaced with a gas engine. The primary advantages of using a gas engine are:

  • Significant operating cost reductions and
  • Fuel diversity and
  • Better part load efficiency than an electric motor

With gas engine applications, customers will be able to achieve a flexibility for processes that weren’t an option just a short while ago. They will be able to “shape” their energy profile, which will allow them to negotiate much more attractive energy prices than if they were committed to a single technology/energy source.

Gas engines can help customers operate more efficiently, more cost effectively and they can offer customers fuel diversity for their facilities.

Natural Gas Engine basics

When identifying an opportunity for, and selling a customer on a Natural Gas Engine application you should be familiar with the technology. This includes knowing not only the benefits of installing a Natural Gas Engine but being able to inform the customer of some facts about the technology and about the maintenance considerations.

A Natural Gas Engine is similar to the engine in your car except that instead of using liquid gasoline as a fuel source, natural gas is used as the fuel source.

There are two types of Natural Gas Engine technologies available for customers, spark ignited Natural Gas Engines and compression ignition Natural Gas Engines.

Spark ignited

  • Use a spark plug to ignite fuel (just like a car engine)
  • Are the most common type of Natural Gas Engine available. They are used in a wide variety of Natural Gas Engine applications. This includes Natural Gas Engines for vehicles, water pumping systems, compressors and chillers.
  • Used in dedicated Natural Gas Engines or natural gas/ propane blend engines
  • Can use gas with a lower Btu content (i.e., sewage plant digester gas)
  • Has a good compression ratio (about 9.4:1)
  • Wide choice of sizes (49 Hp to 2600 Hp)

Compression ignition

  • Commonly seen in heavy-duty transportation applications such as trash haulers.
  • Uses a small charge of diesel fuel to ignite cylinder charge
  • Designed for heavy duty, high load applications
  • Extremely long life
  • Has a higher compression ratio (15:1)

Other technical information

I. Low speed Natural Gas Engines (< 1400 RPM):

  • Typically are larger engines than a high speed engine
  • Longer time between maintenance intervals (rebuilds)
  • Higher first cost due to greater bulk of the equipment

II. High speed Natural Gas Engines (> 1400 RPM):

  • Lower first cost than a low speed model
  • Shorter time between maintenance intervals (rebuilds)
  • Usually turbocharged

III. Automotive derivative Natural Gas Engine:

  • Very low first cost
  • Maintenance, operation and engine life similar to an automobile engine
  • When engine is at end of service life, simply replace with a new unit

I. Rich burn engines:

Engines that use a rich air-fuel ratio. These are common in Southern California because their emissions are very easy to control.

II. Lean burn engines:

Engines that use a lean air-fuel ratio. These engines are not as common as rich burn engines because they require a more sophisticated emission control system.

Other considerations for Natural Gas Engine systems:

Air Quality – Natural Gas engines may need to be permitted and may have emissions limits. It is important to be aware of this when marketing a gas engine system to customers.

Maintenance – Natural Gas Engines do require maintenance, just like a car engine. However, with a maintenance schedule that is strictly adhered to, a Natural Gas Engine will give years of reliable service.

Noise – Natural Gas Engines are louder than an electric motor and may require sound attenuation.

Ventilation – combustion processes require air to take place. Natural Gas Engines need an adequate supply of air to operate. If a central plant is being built or designed, this fact should be considered in the early stages.

How a Gas Engine Works

A gas engine converts the chemical energy of natural gas to mechanical work. The rate of energy conversion in a gas engine is as follows:

100,000 Btu’s into engine

Shaft Work – 31,000 Btu’s (approximately)

Jacket water & lube oil – 35,000 Btu’s (45% recoverable)

Exhaust – 28,000 Btu’s (45% recoverable)

Radiation losses – 6,000 Btu’s (not recoverable)

The energy balance above illustrates why heat recovery adds significantly to engine system efficiency. Usable heat can be recovered from both the jacket water and exhaust. Heat from the jacket water can be used to make hot water, heat from the exhaust can be used to make hot water or low pressure steam.

Air/Fuel system:

Natural gas engines can be naturally aspirated or turbocharged. In naturally aspirated engines air is drawn into the engine at atmospheric pressure. Turbocharged engines use the exhaust to drive a small turbofan that compresses the intake air. The turbofan compresses the air fuel mixture so more molecules are squeezed into the cylinder. When the mixture is ignited, more energy is released. Thus, a turbocharged engine will provide more shaft work out than a naturally aspirated engine of the same size.

turbo_engine

Schematic of a Turbocharger

Turbochargers usually have a heat exchanger (an intercooler or after cooler) located after the compressor fan to remove heat from the compressed air. By cooling the compressed air, the density is increased, so oxygen content for a given volume of air is increased and more fuel can be burned.

turbocharger

Turbocharger with Intercooler

The advantage of a turbocharged engine is that about 35% more work can be done by a turbocharged engine as compared to a naturally aspirated engine of the same size.

turbopic

Gas engine with turbocharger

For more information:

Interested in learning about the parts of a gas engine? Want to understand how a gas engine works? Go here: http://www.howstuffworks.com/engine.htm

Gas Engine Maintenance

Gas engines require regular maintenance just like the engine in your automobile. In fact, the maintenance activities are similar to the maintenance activities you do on your car. Customers with Natural Gas Engine systems can sign a maintenance agreement with their vendor at a reasonable cost.

A good rule of thumb for annualized maintenance costs is $.011 (one and one tenth cent) per horsepower-hour. This amount will vary depending on how the engine is used but is a very good ball park figure to use to determine annualized maintenance costs.

Example: If you have a 200 Hp engine that operates for 2500 hours a year the annualized maintenance costs would be:

200 Hp x 2500 hrs/yr. x $.011/Hp-hr. = $5500

Maintenance is scheduled by the run hours on the engine. A good guide for gas engine maintenance is in the table below:

Daily:

  • Check fluid levels and bring them to proper level (if needed)
  • Visually inspect engine for leaks and/or loose belts and hoses

250 hours:

  • Visual inspection of hoses and belts

750 hours:

  • Oil and filter (air and oil) change
  • Adjust valves
  • Check ignition timing
  • Visually check spark plugs
  • Check/replace fuel filter (the filter cleans moisture and dust from fuel)
  • Clean Crankcase breather

1500 hours:

  • Check/clean/(or replace as necessary) spark plugs
  • Check gas pressure to carburetor (Service Tech can provide this service)
  • Check carburetor adjustment for excess O2 (Service Tech can provide this service)

6000 hours:

  • Exchange/rebuild water pump and water pump idler pulley assembly
  • Inspect Turbocharger and aftercooler (do maintenance per Mfr.’s recommendation)
  • Check cylinder compression (compression should be uniform across cylinders)

Top end overhaul:

Remove cylinder heads and do a valve job and top end inspection at around 11,000 hours (+/-) for both low speed and high speed engines. (Customer should follow Mfr.’s recommendations!)

Note: as valves are cycled in the combustion process they may not seal properly due to wear on the valve and seat surfaces. A valve job refinishes the surface of the valve and the valve seat so that it seals properly when the valve is closed.

Major overhaul:

Top and bottom end rebuilds (valves, cylinder rings, crankshaft and camshaft bearings, oil seals) (at around 24,000 hours for high speed engines; around 34,000 hours for low speed engines)

Other maintenance

Engine oil analysis:

Chemical / physical properties of oil – evaluating oil integrity, checking for water or coolant in the oil and checking for entrained solids. Oil properties can give a very good idea of how the engine is performing and may alert customer to a problem before it becomes a major problem. Customer should check with Mfr. about this service.

Note: regarding maintenance, to convert maintenance hours to something your customer would understand, simply multiply the hours by 50 or 60 to get a mileage equivalent.

For example, the Mfr. recommends changing the water pump at 6000 hours of operation. If you drove your car for 6000 hours at 50 mph, you would accumulate approximately 300,000 miles on your odometer! This gives the customer a better feel for the maintenance requirements of a gas engine.

Air Quality for Gas Engines:

Natural Gas engines may require Air Quality permitting if they are installed. The exception to the permitting requirement is for Agricultural customers (farmers) who use them for Agricultural water pumping for crops or to supply water to livestock.

Most Air Quality Districts have similar emissions limits. For the South Coast Air Quality Management District the limits are:

  • Natural Gas engines of 50 Hp or greater require permitting. (customers in RECLAIM may need to buy emissions offsets to operate an engine)
  • Emission requirements for gas engines are (per Rule 1110.2)NOx – .15 grams per Hp-hr
    CO – .6 grams per Hp-hr
    VOC – .15 grams per Hp-hr
  • Gas engines 1000 Hp or greater require Continuous Emissions Monitoring (CEM)
  • Agricultural engines are exempt from air quality regulations

Emission controls:

Gas engine emissions are very easy to control. For rich burn engines, a non-selective catalyst with an air-fuel ratio controller can be used to keep the emissions below the limits. A non-selective catalyst is similar to the catalytic converter on your car.

Lean burn engines require a selective catalytic reduction system, typically ammonia injection, to clean emissions. Selective catalytic reduction systems are more costly to buy, operate and maintain.

Gas engine Features and Benefits:

  • Gas engines offer very favorable operating cost advantages
  • Gas engine systems are not schedule sensitive. Gas rates have no excessive “demand” charges. Rates are declining block type rates, “the more you use, the less you pay”.
  • Gas engines have variable performance characteristics that match energy use with system demands. Gas engines have very good part load performance.
  • Gas engines offer fuel diversity. Hybrid systems give the customer a choice of fuels. Hybrid systems also can help a customer negotiate better energy prices from suppliers since they can shape their “energy profile” any way they want.
  • You can also use Propane as a back up fuel in the event of Natural Gas supply shortages.
  • Customers can store gas for fuel supply redundancy.
  • The Gas Company can help with permitting of new gas engine technologies.
  • Customers with gas engine or hybrid systems will not feel the effects of brown outs or energy price spikes as much as other businesses. They will have options!
  • Gas is delivered via The Gas Company’s system. There is no need to wait for fuel deliveries as is necessary for propane systems. There are no fuel tanks, which mean no fire code regulatory hurdles or insurance worries. Customers only pay for the natural gas they use, they don’t have to pay for fuel deliveries prior to use so they don’t tie up capital in a tank.
  • Gas engine manufacturers and dealers can provide turn key maintenance packages for customers allowing them to concentrate on their business.

On the next several links we will review benefits of gas engines for specific applications.

Conclusion

Natural Gas Engines offer customers many options for their business and also offer many benefits to customers that choose to make a Gas Engine an integral part of their process.

Superior part-load performance: Gas engines operate much more efficiently at part load than electric motors. The energy consumption will closely follow the load on a gas engine. Electric motors have very, very poor part load efficiency. They consume about 95 – 98% of full load power even at low loads.

part_load_efficiency

perf_efficiency_graph

More stable energy prices: Gas engines often have better operating economics than an electric motor. This is particularly true during warm weather when heavy demand charge penalties kick in.

Gas engines are high-tech: New gas engine packages have advanced microprocessor controls that continuously monitor engine conditions and alert operators of any items that may need attention. They can be used in systems that have centralized controls.

Higher potential total efficiency: Gas engine systems can incorporate heat recovery into the process that raises the overall efficiency to well over 50%. Traditional electric plants have an efficiency of 30%. By including heat recovery in a process savings are achieved on the shaft work and in the recovered heat.

turbo_intercooler

Natural gas has an Octane rating of 130. Octane rating is a measure of the antiknock property of a fuel, it can be thought of as resistance to detonation. High compression engines require high-octane fuels to operate properly and natural gas fits the bill.

Natural gas is free from liquid hydrocarbons. Liquid hydrocarbons, such as Gasoline, Ethanol and Diesel, wash the lubricant from the cylinder walls and dilute engine lubricants. They also leave gum and varnish deposits. Natural gas does not dilute lubricants or leave gum and varnish deposits. Some customers have experienced longer times between oil and filter changes because Natural Gas burns so clean.

Natural gas burns cooler and cleaner than other liquid fuels such as diesel and propane. There is less contamination of the lubricating oil with carbon and nitrates, which helps keep the equipment operating at top performance.


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