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Maximize Fuel Efficiency - Fuel Efficiency 101 - Tech

Posted in How To on July 1, 2009 Comment (0)
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Maximize Fuel Efficiency - Fuel Efficiency 101 - Tech
Photographers: Kevin Blumer

A dozen years ago, on behalf of Chevrolet/Geo, I guided two Trackers over the Rubicon Trail. Both rigs had aluminum underbelly cladding. Steve Kramer, co-owner of Calmini Products, prepped one of the vehicles to serve as a lead and assist rig. A Warn winch, 2 1/2-inch lift kit, and locking rear axle device enabled this two-door Tracker to help a virtually stone stock model through the roughest stuff.

Which one of these vehicles gets the best fuel efficiency? The Dodge Ram weighs 7,800 pounds empty, and these Wranglers weigh 3,400 pounds trail ready. A turbo-diesel powered truck, this unloaded Ram averages 24.3 or more mpg highway. A turbo-diesel engine is much more fuel-efficient than a gasoline engine. The Ram gets better off-pavement fuel efficiency, too. Consider a modern, Which one of these vehicles gets the best fuel efficiency? The Dodge Ram weighs 7,800 pounds empty, and these Wranglers weigh 3,400 pounds trail ready. A turbo-diesel powered truck, this unloaded Ram averages 24.3 or more mpg highway. A turbo-diesel engine is much more fuel-efficient than a gasoline engine. The Ram gets better off-pavement fuel efficiency, too. Consider a modern, "clean tailpipe" turbo-diesel for your next rig. The turbo compensates for altitude, too!

Coming down the Sluice Box into Rubicon Springs after 18-plus hours of driving and winching, my support vehicle was nearly out of fuel. The modified lead vehicle had consumed twice as much fuel as the stock vehicle! The reason was clear enough - off-pavement on the rocks, "mileage" is not measurable. Fuel consumption is the product of 1) engine running time, 2) load, and 3) the engine's operating rpm.

Your off-road driving habits have a strong influence on the amount of fuel consumed. If you hold the engine at a higher rpm in low-range and the lower gears, economy suffers. For saving fuel off-highway, use the gear that enables adequate pulling power, reasonable compression braking and the least rpm necessary. It may be prudent to be in second or even third gear of low range, with the engine speed low, rather than in first gear with the engine revving high. Use of a higher transmission gear with light throttle adds the benefit of less torque available at the wheels - this is a plus when you want to reduce tire spin. You can always shift down a gear for better compression braking on down slopes or for rock crawling.

EFI's overall goal is to keep the engine running leaner or as near EFI's overall goal is to keep the engine running leaner or as near "stoichiometric" as possible at cruise speeds or under normal loads - this is 14.7:1 air/fuel for the optimal burn of gasoline. Accelerating or heavy engine loading requires a richer mixture, which the system can also create as needed. The 4.0L engine delivers 20 mpg at highway cruise, retrofitted into an '87 YJ Wrangler chassis to improve off-road efficiency. Transmission is a freshly rebuilt AX-15 with overdrive, adding to good highway mileage.

Engine Prepping for the Trail
For a carbureted engine, severe altitude changes can require disassembling the carburetor and making internal jet changes. Some carburetors are sensitive to altitude changes of 1,000 feet or less. In the early '90s, I performed detailed air-fuel ratio testing on various Holley 2300 carburetors fitted to Jeep 4.2L sixes. For some carburetors, the jets needed changing at altitude fluctuations as slight as 500 to 800 feet! Such an engine tuned for sea level will run very rich at high altitudes. Similarly, high altitude tuning will create a very lean burn at sea level. Lugging a rich burning engine can foul spark plugs and cause misfire. Lean burning will cause detonation (ping) or, worse yet, pre-ignition and piston damage.

These problems were partially remedied by electronic ignitions that provided higher firing voltage with wider spark plug gaps. In the '80s, clean air standards led AMC/Jeep and other manufacturers to use feedback carburetors. These carburetors had primitive means for altitude compensation. In the '80s, module-and-feedback ignition distributors achieved more complete fuel burn. The '91 Grand Wagoneer was the last carbureted Jeep engine.

Modern fuel injection readily compensates for altitude and changes in atmospheric pressure. The engine management computer constantly adjusts and stabilizes the air-fuel ratio, whether at sea level, atop the "fourteeners" in the Colorado Rockies, or inching over the Himalayas. (At cruise and light loads, the optimal or "stoichiometric" air-fuel ratio for gasoline is 14.7:1.) With the capacity to adjust spark timing electronically, EFI contributes to the efficiency and predictable behavior we enjoy in modern engines!

Piston travel and throttle position determine the amount of fuel consumed off-road. Using low range and lower gears, consider how many feet - or miles - the pistons travel up and down in their cylinders. This determines the amount of fuel burned, and running time becomes more important than miles traveled. Winch loads and fast idling demand fuel as well. Piston travel and throttle position determine the amount of fuel consumed off-road. Using low range and lower gears, consider how many feet - or miles - the pistons travel up and down in their cylinders. This determines the amount of fuel burned, and running time becomes more important than miles traveled. Winch loads and fast idling demand fuel as well.

So, what is a trail "tune-up" for the modern fuel-injected engine? After a fresh oil filter and oil change, a tune-up, much like carbureted engines with conventional ignitions, is restorative. Despite the greatly extended service intervals on items like spark plugs, plug cables and the latest coil-on-plug distributor-less ignitions, parts wear and fatigue are still an issue. Spark plugs, even platinum, palladium and nickel types, do have a lifespan. Spark plug cables face deterioration along with the distributor cap and rotor. True, that interval may be longer (our '99 XJ Cherokee had what looked like the original spark plugs, cables, cap and rotor when we bought the vehicle at 94,000 miles!). An improperly maintained engine will exhibit poor fuel efficiency, higher operating temperatures, and poor performance under load.

The milestone breakthrough for altitude and fuel metering was fuel injection with electronic fuel-and-spark management. Jeep was among the earliest 4WD platforms to use EFI. Here is the 1987 YJ Wrangler 2.5L four-cylinder with TBI. The feedback sensors, including an oxygen sensor and manifold pressure sensor, enabled the electronic control module to instantaneously adjust fuel flow and spark timing to meet various engine loads and atmospheric conditions. The milestone breakthrough for altitude and fuel metering was fuel injection with electronic fuel-and-spark management. Jeep was among the earliest 4WD platforms to use EFI. Here is the 1987 YJ Wrangler 2.5L four-cylinder with TBI. The feedback sensors, including an oxygen sensor and manifold pressure sensor, enabled the electronic control module to instantaneously adjust fuel flow and spark timing to meet various engine loads and atmospheric conditions.

Spark plugs are relatively cheap-even the highest quality plugs run no more than the current cost of 3-4 gallons of unleaded regular fuel. New spark plugs can restore power and fuel efficiency, preserving an engine while earning back the parts cost through fuel savings. As a rule, replace spark plugs at no more than 30,000 miles on an EFI engine, 15,000 or less on a carbureted engine. Unless your vehicle uses extended life (100,000-mile) type spark plugs, the 30K figure should work for EFI inline sixes, V-8s and all fours. V-6s like the Liberty's 3.7L - and other engines that require gymnastics to change the plugs - can follow factory-recommended service intervals.

If your 4x4 has a carburetor, this is a periodically serviced item. The legendary woes of a Carter BBD two-barrel on 258 Jeep inline sixes can often be eliminated by a blueprint rebuild of the carburetor. Expect to rebuild the carburetor each 30,000 to 40,000 miles, more often if you operate the vehicle in dusty or rock crawling conditions - or if your fuel tank is aging...If the fuel tank condition is questionable, service or replace the tank!

EFI/MPI can keep the fuel mix stable at all altitudes by constantly reading the oxygen content of the exhaust and setting the injector fuel flow accordingly. Note that while the fuel mix is correct, there is still a loss of horsepower, approximately 3- to 5-percent per 1,000 feet of elevation above sea level - another reason to use low range at higher elevations! EFI/MPI can keep the fuel mix stable at all altitudes by constantly reading the oxygen content of the exhaust and setting the injector fuel flow accordingly. Note that while the fuel mix is correct, there is still a loss of horsepower, approximately 3- to 5-percent per 1,000 feet of elevation above sea level - another reason to use low range at higher elevations!

This raises the issue of filtration, and adequate fuel and air filtration is crucial for 4x4s. If you have a model with inline fuel filter(s), replace them according to the factory service interval. Frequency for changing an air filter element varies. The factory recommended interval is the longest you should leave the filter in place. For any trail-driven 4x4, the air filter's condition depends upon the dust level of the trail or road you've just traveled! There are filters that clogged in a dozen miles of the Rubicon Trail or one trip down an alkaline, corrugated dirt road in the desert. Always carry a spare air filter, and when traveling to a remote area, consider carrying a spare fuel filter, too. On any model, be certain that the fuel cap seals properly and keeps dust from entering the fuel tank.

Conserving Fuel at All Cost
There are instances when off-road fuel pinching is crucial. Fuel may be running low, or the trail time is ticking away faster than anticipated. For the two Geo Trackers, strap towing and long idling used up all the fuel in the support vehicle and just a bit more than half a tank in the other! Beyond selecting the correct transmission gear and monitoring throttle use, there are other ways to conserve fuel off-pavement.

In some instances, it is wise to winch instead of drive through a stretch. Examples would be deep sand, mud or snow - or maybe a swift creek with a questionable bed. Spending hours spinning wheels churning through muck or inching out of a snow pack could be a colossal waste of fuel. Use of a winch, a Pull-Pal anchor, a Hi-Lift jack, and other accessories could save time, fuel, and energy. A winch uses current from the battery, and the load on your engine becomes nothing more than the speed to keep the alternator charging. If you have dual batteries and an isolator, you may be able to winch through an obstacle without running the engine.

Keeping tires, brakes, wheel bearings, U-joints, belts, and hoses in top shape will lessen the need for trailside fixes. Be certain that all fuel hoses and pipes are in top condition and out of harm's way. An unnoticed fuel leak off-pavement could quickly lead to trouble...avoid running the engine to provide work lights at night. Bring portable lamps and plenty of layered, warm clothing and blankets. If you need to stop for a while in cold weather, avoid idling the engine to keep the heater going.

Extra fuel should always be available when traveling remote trails. These days, you'll want to fill the cans at an area with reasonable fuel costs. (Fuel as high as $6 per gallon is possible in a remote or mountain community; locks on fuel cans are advisable.) Keep in mind that off-road fuel consumption is time based. If a road looks primitive on the map, make sure you know how many hours you will drive - not the distance. Be aware of your vehicle's fuel habits in low range and at crawl speeds!

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