The engine of the future is probably similar to the one in your current truck. It's an internal combustion engine with pistons, valves, crankshaft and other familiar parts, an inline 4 or 6, or a V-6 or V-8.

Those popular configurations have been in use for many years and will likely remain most common for some time.

The more commercially oriented the truck, the more likely its engine is an inline or "straight" design. Its block is stiff and strong, and it's easy to make and simple to maintain. All kinds of advanced fuel, air and electronics systems can be applied to it.

Since the 1930s, when high-speed truck diesels began appearing from Cummins and Caterpillar, the design used has mostly been the inline six. Detroit Diesel sold V-6, V-8 and V-12 two-stroke diesels along with a few inline versions for many years, but their main attribute was shorter length in a time of strict vehicle length limits. By the late '80s, Detroit had gone to a four-stroke inline six. The most recently introduced heavy-duty diesels from all major builders here and overseas are all inline sixes.

Lighter trucks, however, are more likely to still have V-type engines, because they're more compact and can fit into crowded engine compartments. And they have a cachet of performance. Henry Ford wasn't the first with a V-8, but he popularized it as a high-performance engine when the output numbers from his own straight 6s were actually a little better. The V-8 caught on in cars and light trucks, and is still with us in the latest gasoline and diesel engines from the domestic Big Three as well as many overseas makes.

Both inline and V-type engines make good power and torque for their sizes and weights. Thanks to many years of refinement, often in response to increasingly stringent exhaust emissions rules, current engines, whether gasoline or diesel, are increasingly efficient. Builders are always looking for something better, and have considered turbines, fuel cells and other engine types. But the piston-type engine remains superior when all things -- economy, durability and acceptance by customers -- are considered.

Look for current designs to remain dominant for another 10 to 20 years ... if nothing better comes along.

What if there is something better? Several things are in the works thatcould offer enough advantages that changing production to accommodate them may be worth the trouble. In the short term, they're likely to be of opposed-piston design. OPs, where two pistons share a single cylinder, are not new. Fairbanks-Morse built hundreds for U.S Navy submarines in World War II, then for its own diesel-electric locomotives in the 1940s and '50s.

A principle advantage of an OP is simplicity: A pair of pistons shares a combustion chamber at the center of the cylinder and fire away from each other, with fuel-air mixture introduced through injectors and ports in the wall, so there are no valves or valve train. Each piston spins a separate crankshaft at opposite ends of the block.

In the last few years, two new companies have been designing modern truck- and auto-sized engines using the OP layout, and with great promise.

Achates Power in San Diego says its OP engine, now in dyno testing, has achieved fuel efficiency that's 21% better than a benchmark V-8, the latest 6.7-liter PowerStroke from Ford (not its Navistar-built predecessor), and meets 2010 emissions limits. Achates engineers have done 2,500 hours of testing with six engines, run in turns between modifications and adjustments. Company President David Johnson says they're "on the steep part of the learning curve for the opposed-piston engine and efficiency should get even better."

In suburban Detroit, EcoMotors International continues work on a variant, an opposed-piston, opposed-cylinder, or OPOC, engine. The basic EcoMotors engine is a module consisting of a single cylinder with four pistons, in two sets of two pistons, one firing against the other, and all linked to a single, central crank.

Like the Achates, the EcoMotors OPOC has no heads or valves, and instead uses internal ports to draw in fresh air and expel exhaust gases. That and its frequent power delivery make it a two-stroke engine. It's fuel-injected and turbocharged, with the turbo moving the air through circumferential ports.

The OPOC engine is smaller, lighter, and more fuel-efficient than anything now out there, says Don Runkle, EcoMotors president. It has two to three times the power density of conventional diesels with half the parts, which makes it 20% less costly to manufacture. And it has at least 15% better fuel efficiency.

Its small size would make it easy to "package" in many vehicle types. That intrigues Navistar International, which has inked an agreement with EcoMotors and hopes to have a running truck in a few months, and OPOC-powered trucks for sale in two to three years. Navistar is a longtime maker of conventional diesels, so its publicly expressed hopes say a lot for this engine's promise. Meanwhile, EcoMotors is working on other deals with other builders, as is Achates.

Then there's the Scuderi Split Cycle motor from the Scuderi Group in West Springfield, Mass. It's a variation on the old Miller-cycle theme, and has achieved significant fuel savings in tests so far.

The Scuderi engine divides the four strokes of a combustion cycle between two paired cylinders. The left cylinder functions as an air compressor, handling intake and compression, while the right cylinder handles combustion and exhaust.

Key to Scuderi's split-cycle design is that it compresses the air before it fires. By optimizing the split-cycle concept, the engine when fully developed will reduce NOx emissions up to 80% and improve fuel efficiency by as much 50% compared to a conventional gasoline engine. The engine requires one crankshaft revolution to complete a single combustion cycle and is projected to have higher torque, better thermodynamic efficiency, and lower emissions than possible with today's engines.

Hydrogen fuel-cell drayage tractors are now running out of the Ports of Los Angeles and Long Beach. They're made by Vision Motor Corp., which installs the fuel cell, electric motors and electronic control systems in Freightliner Columbia daycab tractors.

The nearly silent powertrain claims an output of up to 536 horsepower and 3,300 pounds-feet, with torque limited to protect its gearboxes. The hydrogen fuel makes the Tyrano tractor 36% less costly to operate than straight diesel and 50% less than natural gas, which is currently used by several hundred tractors in the SoCal ports to help curb air pollution.

A hydrogen fuel cell produces electricity when it contacts water along a special membrane. The only exhaust is water vapor, say firms that have been working on them. The price of each Tyrano is not clear, but about $80,000 in federal and state money subsidizes the buying of each one, according to one report. Total Transport, a drayage operator in the Los Angeles Basin, has ordered 100.

Hybrid drives -- usually using electric propulsion but also hydraulic mechanisms -- have in the first 10 years of this century been paired with piston engines. Eaton builds an electric system that's sold by Freightliner, Kenworth, Peterbilt and Navistar, each using its own diesel to share propulsion duties, in Class 6 and 7 trucks. Eaton also makes the Hydraulic Launch Assist mechanism for use in heavy trash collection trucks.

Azure Dynamics works with Ford to market an electric hybrid system it installs in E-450 chassis using Ford's gasoline V-8s.

Hybrids have steadily gained customers. Government funding has been needed to make a decent business case for these expensive products, but this is changing as prices for batteries and other components hav