Q: Why does APU fuel consumption differ, when the manufacturers all use the same size engines?
A: The most popular engine type for current Auxiliary Power Unit (APU) applications is a two-cylinder diesel motor. Fuel consumption by these “mini” power stations is directly proportionate to the sustained engine rpms and load required by the APU to properly perform.
APU heating, cooling and in-cab power generation functions require different engine loads. Like with any car or truck, the harder your engine works the less fuel economy you’ll get. Contributing to varying APU engine loads are components like the cooling fan (pitch), generator size and load, AC compressor and alternator for determining how hard its engine works.
The best fuel economy from an APU is likely found within the heating function alone. However, running the air conditioning plus drawing 30-40 amps on the generator for in-cab appliances is a different story. In this case, the APU motor will work harder and sip more fuel to drive the engine speeds to produce the right frequency Hz for maximum generator load.
Smaller generators translate into less engine loads, so why not run three cylinder motors? Some manufacturers do. The bigger engines reduce rpms required and engine noise but likely use the same or more fuel, cost more and require more space. If you want to run the refrigerator, microwave, television, satellite dish, interior lights and AC, some APUs like our PowerCube™ are designed for that; sipping just 0.4 gallons per hour under maximum load. With heating alone, fuel consumption on the same unit drops to 0.165 gallons per hour.
Finally, ask for clarification specs on the size of the components used within your choice of APU system, and how the engine load reflects reported fuel consumption numbers listed.
Q: Why do some APU’s run louder than others?
A: I recognize that a new or different noise or vibration is not something a driver is going to warm up to, but having said that, the “product acceptance” curve on APUs is gaining. The process not only includes enjoying benefits such as fuel savings, reduced emissions and in-cab comforts, but drivers are adjusting to living with new sound frequencies, which influence noise levels, extra weight and more maintenance.
In the early stages of most mechanical evolution, there eventually comes a need from the market to build something better, and APU’s are no exception. Performance expectations on early APUs were low, because it was a new concept. “Loud” wasn’t an important term and didn’t really matter because there was nothing else to compare with, and most drivers were infatuated with getting some level of AC and heat. Early expansion of the APU market generally meant that new designs evolved from the first generation and so on. This is where many APU customer expectations were formed.
Drivers are having a difficult time with the evolution of the APU, because their expectations are drawn from the older designs. They want an APU that stands up better to heavy service, and has better HVAC capabilities and generates more power. Integrated, battery based and independent APU systems have differing noise levels because of the size and number of components they drive.
APU systems like PowerCube generate slightly higher decibel levels than an integrated or battery based system because the engine is responsible for operating more integrated and larger components to sustain cab temperatures and voltage output.
The basic fact is that decibel levels of an APU generally increase with higher performance configurations. Better, more sustainable air and power do not equal a quieter-sounding APU in operation. That high performance equation does compute operating smaller components and less rpms.
So, there is certainly a give-and-take on performance preference, but an APU configuration having low noise along with better performance in not a viable near-term scenario in this mechanical configuration.
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