As it turns out, there are very few technological barriers to overcome. In fact, converting existing vehicles to burn natural gas isn't particularly challenging. Unfortunately, if you tried to do it yourself, you'd more than likely run afoul of the Clean Air Act's rules against modifying fuel systems--a violation that could cost you up to $5000 in fines for every day you drive the converted vehicle. So if you want to green your wheels today, the only way to do it is by hiring a certified compressed-natural-gas (CNG) installer to do the job. To get the skinny on aftermarket CNG systems, I visited NatGasCar in Cleveland. It's a startup shop that augments gasoline cars by installing a parallel natural gas fuel system. They showed me their latest creation, a dual-fuel Dodge Caravan intended for airport taxi service. It starts on gasoline and switches over to natural gas once the engine warms up.
NatGasCar's biggest component is also its most crucial and expensive--the compressed-natural-gas fuel tank situated behind the rear seats in the cargo area. The company uses a Type 4 tank, the most advanced kind. It reduces weight with a plastic composite core wrapped in carbon fiber and is rated for severe impact and puncture resistance.
Between the tank and the engine is the fuel regulator, which reduces the fuel-tank pressure of 3600 psi to a usable 125 psi delivered to the engine. The fuel regulator is heated to prevent freezing from the expansion of the gas. The lower-pressure gas travels to the engine, Chrysler's flex-fuel-capable Pentastar V-6. A flex-fuel engine is important, since it has hardened valves and valve seats, which are necessary for CNG operation. The natural gas is routed through a parallel fuel rail, and a second set of injectors is plugged into a clever adapter designed to accommodate both the gasoline and CNG injectors on the same injection port. Natural gas runs at an ideal air--fuel ratio of about 16.8:1, whereas gasoline runs happily at 14.6:1 for the Pentastar engine. As a result, the programming for the new injectors has to be slightly different. NatGasCar's wiring harness intercepts the signals from the engine-control module and, depending upon which fuel is selected, turns on either the gasoline or the CNG injectors. The signals bound for the gasoline injectors are modified to deliver the appropriate amount of fuel to the natural gas injectors. This way, very little fine tuning is necessary, and the car's engine-control unit does most of the work.
Fueling Woes
So a new fuel tank and a little bit of tinkering with the fuel injectors and I'm ready to go, right? Unfortunately not. Natural gas is delivered across the country to millions of homes. But what would seem to be the ideal distribution network is actually the biggest headache of natural gas vehicles. Home natural gas is delivered at about 0.5 psi, but natural gas in vehicles needs to be pressurized to 3600 psi. So if you want to use CNG in your car, you'll need a compressor. A National Fire Protection Association safety standard bans compressed gas storage in homes, so a stand-alone multistage compressor pump in the garage must be hooked up to the vehicle's fuel tank, filling it directly. This leads to fueling times of up to 22 hours (even longer than equivalent home charging times for electric vehicles). Honda's Civic Natural Gas is paired with a home compressor system called Phill ($4500), the only commercially available product of its kind. NatGasCar is developing a compressor system capable of 8-hour fill-ups; the current target price is $3500. Some states have incentivized the installation of high-speed filling systems at gas stations, where fill times are as brief as 4 to 5 minutes, much like gasoline's. But these systems cost $750,000 per station to install, and low demand means there are only 941 high-pressure CNG filling stations scattered across the country, mostly in New York, California, Utah, and Texas.
Economics
Okay, okay, it's a pain to fuel CNG vehicles, but is it worth it? Nationwide, natural gas ranges from 79 cents to $1.50 for a gasoline gallon equivalent (gge) of fuel. That's considerable savings over petroleum-based products, especially considering that CNG vehicles get the same or better relative fuel economy per Btu because of the higher octane rating of natural gas. Our test drives indicated no hit to performance, and a perfectly acceptable range of about 250 miles. But there are some pretty extraordinary initial setup costs. A properly installed conversion will run anywhere from $6500 for a basic system to $12,000 for a top-of-the-line installation with a high-capacity, composite fuel tank. If you want a home fueling compressor, tack on another $3500 minimum. Even at the low end, you're looking at spending enough on the conversion to buy more than 1800 gallons of gasoline at today's prices.
Those prices will ultimately determine the fate of CNG vehicles. High gasoline prices historically have caused furious investment in cheaper, cleaner fuels, followed by a collapse in demand when gas prices fall. For now, CNG has a high price of entry that makes it viable only for taxi services and other fleet operators, but over time, economies of scale may bring down the costs for the ordinary car buyer. And if gasoline stays above $3 a gallon, that change may come sooner rather than later.
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