FAQs
Q. What is natural gas?
A. Natural gas is a mixture of gases that come from the earth. It can come from coal beds, oil reserves, shale veins, etc. It is mostly methane, but usually has small percentages of many other hydrocarbons, such as ethane, butane, propane, pentane, etc. It also can have inert gases such as carbon dioxide, nitrogen, helium, and hydrogen sulfide.
Q. Are propane and natural gas the same?
A. No. Propane can be a component of natural gas, but with propane, you are dealing with a product of refinement. It has been separated from natural gas or oil and is a single product. It is stored as a liquid at a relatively low pressure. Natural gas is normally stripped of the heavier components such as propane, butane, ethane, etc, for several reasons. One reason is that the components are usually worth more money to sell as a separate fuel than to leave it in the natural gas. Another reason is that energy producers need to keep the amount of energy in a cubic foot of gas at 1000 btus. If the natural gas isn’t almost all methane, the energy level will be higher than that and they will have to add air to the gas to bring it down. Otherwise, appliances will not be adjusted properly and can smoke and create soot.
Q. Can I store compressed natural gas in a propane tank?
A. No. Propane tanks are made to handle up to 250 psi, with a relief valve that usually opens at 275-300 psi. Since compressed natural gas doesn’t liquefy, it requires much higher pressure to store enough fuel in the tank to have a reasonable range. CNG is stored at up to 3600 psi, which requires a much stronger tank than propane. CNG tanks are much like oxygen cylinders in construction. Automotive tanks are typically made of steel, aluminum, or carbon fiber. Carbon fiber is much lighter than steel or aluminum and is the tank of choice for vehicles.
Q. Can natural gas be liquefied?
A. Yes. By cooling the gas to -260°F, it will liquefy. It must be kept very cold, otherwise it will begin to vaporize and the pressure will soar. LNG tanks are double wall with insulation. There are very few refueling stations in the U.S. for LNG, with most of it being used for municipal buses and large trucks. Since it has to remain cold, LNG needs to be used very soon after the tank is filled. It doesn’t lend itself to personal vehicles for this reason. Because of being cryogenic, it has freezing hazards at fueling stations.
Q. Which is more practical for vehicles, propane or CNG?
A. It depends upon how you define practical. Since propane is stored as a liquid, it is a much more dense energy source than CNG. A 10 gallon propane tank takes up the same amount of space as a 10 gallon gasoline or diesel tank. Propane has 91,000 BTU/gallon, so it isn’t as energy dense as gasoline or diesel fuel, but it is a lot more dense than CNG. A 10 GGE tank of CNG is approximately the same size as a 35 gallon gasoline tank. Therefore, range is the issue. Most vehicles have limited space for fuel tanks, so it is difficult to carry a large amount of CNG as opposed to propane. From a storage aspect, propane makes more sense for a road fuel. Cost per BTU is much less for CNG than propane. If a gallon of propane costs $2.00, and a GGE of CNG costs $2.00, the CNG is a better value because you are getting 125,000 btus instead of 91,000 for the same price. Typically, CNG is about half the price per btu of propane at the retail level.
Q. How much can running CNG or propane save me?
A. This all depends on what proportion of gas to diesel that you run, and the cost difference between the gas and diesel. Propane has 2/3 the energy per gallon that diesel contains, so it will take three gallons of propane to replace two gallons of diesel. Therefore, If propane costs 2/3 the price of diesel fuel, there will be no savings, because they are actually the same cost per btu. The key to saving money with either propane or CNG is the difference in cost per btu. Natural gas typically will be much less than propane per btu.
For example, let's say you have a diesel pickup truck that gets 20 mpg. Diesel fuel has around 140,000 btu per gallon. This means that you use about 7000 btu/mile. When you run diesel and gas, you will still consume 7000 btu/mile, but it is no made up of both fuels. The goal is to use as little of the expensive diesel fuel and as much gas as is practical. Let's say that with the gas system in use, you now get 50 mpg on diesel fuel. You are now using about 2800 btu/mi of diesel fuel. This means that 4200 btu are coming from the gas. If burning propane, wihich is 91,000 btu/gal, you should be getting around 21 mpg on propane. If burning CNG, which is 125,000 btu per gasoline gallon equivalen (GGE), you should be getting around 29 mpg. If diesel fuel is $4.00/gal, running diesel only has your operating cost at 20¢ per mile. While achieving 50 mpg, the diesel cost per mile drops to 8¢/mi. Let's say you can get propane for $1.75/gal. The propane cost/mile would be 8.3¢/mi. Add the two, and you are at 16.3¢/mi, for a savings of 3.7¢/mi. If using CNG at $1.75/GGE, your cost per mile of the gas would be 6¢/mi. Add the diesel at 8¢, and you have 14¢/mi, for a savings of 6¢/mile.
Q. How is CNG sold?
A. Since CNG isn’t liquefied, it can’t be sold by the gallon or liter. However, most people are used to dealing with these units of measure because of gasoline and diesel. So, CNG dispensers sell gas by the Gasoline Gallon Equivalent, or GGE. One GGE contains the same amount of energy as one gallon of gasoline. A DGE is the equivalent amount of energy as a gallon of diesel fuel. Normally on a dual fuel gasoline/CNG vehicle, the vehicle will go approximately the same amount of miles on a GGE of natural gas as a gallon of gasoline. One gallon of gasoline has approximately 125,000 BTU of energy. This is approximately 125 cubic feet of natural gas. Therefore, a 10 GGE natural gas cylinder would contain about 1250 cubic feet of gas when filled to 3600 psi.
Q. Where is CNG sold?
A. CNG is sold at a number of stations throughout the United States and the world. You can find a map of all the stations close to you at this site... www.cngprices.com
Q. I can't find any CNG stations near me. Can I still run CNG?
A. Maybe, if you have access to pipeline natural gas. If you heat your home with natural gas, you can run a compressor that will allow you to fill the vehicle tanks. Compressors are quite expensive, but the more fuel you burn, the more you can save. We handle a line of compressors that start at 1.4 GGE per hour compression rate. Click here for more information.
Q. Can CNG or propane hurt my engine?
A. Yes, if run in too high of quantity. Two things can happen. One is overheating the valves and pistons from overworking the engine. The other is from preignition. The overheat occurs because the engine is capable of making more power running gas, and if that power is used, high exhaust temperatures will result. This is easily controlled by backing off the throttle, or by installing an exhaust temperature gauge with automatic gas shutoff above a user settable temperature.
Preignition comes from using so much gas that the air fuel mixture becomes rich enough to ignite under compression, rather than waiting to be ignited when the diesel fuel is injected. The result is an increase in engine noise, and depending on the severity, damage to pistons. Preignition is much easier to have happen with propane, because it has a lower octane rating than natural gas. Most applications don't have any issues with preignition with our natural gas mechanical system, because it simply can't deliver so much gas to induce it. With propane, we recommend adjusting the flow low enough to avoid it. Our upcoming electronic controller will sense detonation and automatically curtail gas use to avoid it.
Q. Can natural gas or propane be used in a stationary diesel engine?
A. Absolutely. Let's discuss natural gas first. Stationary applications have a very fast return on investment. Much of the cost of running natural gas on vehicles is compression and storage. This isn't required on an engine that can have a gas line run permanently to it. Stationary engines typically run constant speeds, so it is easy to run a high substitution rate. Depending on load, we have engines running as much as 80% natural gas. Even very heavily loaded engines can usually run at least 50% gas.
Typically, commercial gas customers buy the gas by the MCF. This is 1000 cubic feet, and around 1 million btus. This is equivalent to the energy contained in 7 gallons of #2 diesel fuel. If you have an engine that normally burns 14 gallons of diesel fuel per hour, and you now run 50% natural gas, the diesel consumption drops to 7 gal/hr. The gas consumption would be the equivalent of 7 gal/hr, which would be 1 MCF/hr. If the commercial rate for the gas is $7.00/MCF, then you are replacing diesel fuel with gas at the equivalent of $1.00/gal. So if our example engine runs a 10 hour day, and diesel fuel is $3.50/gal, here is how the savings is calculated:
Straight diesel- 14 gal x 10 hrs = 140 gal/day. 140 gal x $2.50 = $350/day
Running 50/50 natural gas- 7 gal x 10 hrs = 70 gal/day x $2.50 = $175/day for diesel. 1 MCF/hr gas at $7.00 per MCF x 10 hrs = $70/day for gas. $175 + $70 = $245/day. $350 - $245 = $105 per day savings.
Running 70% natural gas would result in 4.2 gal/hr diesel consumption. 42 gal/day at $2.50 = $105/day for diesel. 9.8 diesel gallon equivalents (DGE) of gas would be 9.8/7 = 1.4 MCF/hr gas consumption. 1.4 x 10 = 14 MCF/day x $7.00/MCF = $98/day for gas. $105/day for diesel + $98/day for gas = $203/day total fuel cost. Therefore, the total fuel cost is nearly half of what it was on straight diesel.
The key to savings is the difference in cost per btu between diesel fuel and natural gas. The propane systems work much the same way, but propane is typically higher in cost per btu than natural gas. Propane is sold by the gallon. Propane has only 91,000 btu/gal compared to diesel fuel at 140,000 btu/gal. Therefore, it will take three gallons of propane to replace two gallons of diesel. Let's use our engine example above to illustrate:
50/50 diesel propane- 7 gal/hr diesel fuel, 10.5 gal/hr propane. Let's say propane is $1.00/gal. The daily diesel cost would be $175. The propane would be 10.5 gal/hr x 10 hr x $1.00/gal = $105. $175 + $105 = $280. Savings would be $350 - $280 = $70/day.
Not quite as attractive as the savings with natural gas. This is because the cost/btu is higher. If natural gas isn't available, propane is a good second choice. Another thing about propane is that high percentages can't be run as easily because of ease of detonation since the octane rating is lower than natural gas. This would vary by engine.