Idling: Gentlemen, Stop Your Engines

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At the moment, diesel-powered generators or auxiliary power units (APUs) seem the most practical alternative to idling, but other technologies may soon surpass them for efficiency and practicality. Cost may be another matter.

Freightliner, for example, has been developing a hydrogen fuel-cell APU. Fuelled by liquid hydrogen from a 197-litre tank, the present second-generation unit produces 1.4 kilowatts of 120-volt AC or 12-volt DC power and is connected to the truck’s electrical system by an 1800-watt inverter.

The demo truck has an 8000-BTU/hour air-conditioning unit to show its ability to cool the truck without idling the engine. A fuel cell creates no harmful emissions, its only byproduct is water.

Freightliner says it’s working to make fuel-cell APUs commercially viable within two or three years. Among the issues to be resolved are cost, integration with other truck systems, and the availability of suitable fuel.

Then there’s Caterpillar’s MorElectric, which is nearly ready for market. The engine maker’s electronics division is working on this stunning new way to drive your air conditioner, air compressor, and other accessories from one integrated unit. The company says MorElectric can provide heating, cooling, and accessory power — including battery charging — without idling the engine. Trucks will be equipped to use shore power wherever it’s available, and an optional onboard APU will provide cab comfort where plug-ins don’t exist.

In essence, Cat’s plan is to drive many auxiliary engine components electrically, rather than using gears and drive belts — including various pumps and compressors, like the A/C compressor — even while driving the truck. Through the use of modular electrical components, the Cat system could be used to drive HVAC needs and other modest electrical demands from a storage cell or through shore power rather than using the engine. Among its more interesting aspects is that, without the need to use drive belts or shafts, some components can be moved to very non-traditional locations.

Available right now are other alternatives. Auxiliary power units and on-board generator sets — power inverters as well to a lesser extent — can curb idling substantially. They’re all much easier to cost-justify than they were even five years ago, especially with the price of diesel constantly rising.

Are they really needed? Yep. In Europe it’s illegal to idle your truck, period, even when you’re loading and unloading. Anti-idling legislation is mushrooming here as well, and the fines can be astonishing. A citation in New York City; for example, can set you back between $375 and — sitting down? — $15,000. A second offence can cost 20 grand.

There’s a clear payback to buying an APU or on-board gen set, but it’s easier for some truck operators to bury the price of staying warm in the overall cost of running their trucks than it is to tack another $5,000 to $10,000 onto the price of a new rig. Still, the on-going savings are compelling. Idle a truck eight hours a day, six days a week, 50 weeks a year, and you’ll spend some $43,200 over five years.

Power inverters are the least expensive of all, but are also the most limited in terms of power output. They convert truck-battery power to 120 volts and the better ones can run a microwave and a coffee-maker at the same time.

There are many variations on the APU, but basically they consist of a small diesel engine running a 12-volt battery-charging alternator and maybe an automotive air-conditioning compressor.

The little liquid-cooled diesel is plumbed right into the truck’s cooling system to provide heat in winter. The air conditioning system usually has a separate evaporator unit mounted under the bunk. APUs usually require an inverter to supply 110-volt AC power. The 12-volt alternator supplies a constant charge to the truck’s batteries to overcome the draw from the inverter.

Given the limited output offered by inverters, APUs generally can’t be relied upon to supply enough power for an engine block heater, for example, but since the APU uses the vehicle’s cooling system, the engine will very likely be warm enough to start without a block heater. These systems lend themselves well to the installation of a high-output inverter. Typically they weigh 300-500 pounds and are installed on the frame rails below or behind the sleeper.

The 120-volt AC Generator, also known as gen-sets, consist of a small diesel engine driving a 120-volt AC generator to provide AC power, while a 12-volt DC alternator provides a charge to the vehicle’s battery to keep the charge up if you like to leave the lights on at night. They can be used in conjunction with electrically driven 120-volt A/C compressors or heaters.

Gen-sets are usually not connected directly to the truck’s HVAC system, rather they use their own coolant to provide bunk heat in a closed-circuit system. They can be operated while the engine is running, allowing for continued battery charging and A/C function if the truck’s own systems fail. They’re easily hooked up to shore-power installations too.

Depending on the model and brand, gen-sets can produce 4,000 to 5,000 watts of power and somewhere in the range of 12,000 to 15,000 BTU of cooling, or 15,000 to 20,000 BTU of heat. They draw fuel from the truck’s fuel tanks, eliminating the need for an independent fuel supply. Depending on the frame space available, installation may require moving other frame-mounted components.

Gen-sets offer the greatest functionality and performance, but they cost more than an APU. Frame space demands are about the same, as is fuel consumption and weight. A thorough needs analysis would reveal which option is best for your application.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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