Q: I am curious about the Atkinson cycle and variable valve timing as they relate to efficiency (increased mpg) improvements. The Prius appears to be the only mass-production car that uses the Atkinson cycle. Since the Atkinson cycle is more efficient than the classic Otto cycle, what are the benefits and drawbacks of the Atkinson cycle and why is the Atkinson cycle not in more widespread use? Regarding variable valve timing, how is it performed and how does the variable valve timing in current use impact efficiency?

— Don Van Buren

A: The modern Atkinson cycle differs from the original design but utilizes the same concept of the expansion ratio being larger than the compression ratio. The idea is to improve thermal efficiency by employing a power stroke that is longer than the compression stroke. Modern engines accomplish this with altered valve timing that differs from that used in typical Otto cycle engines. The Atkinson cycle prioritizes efficiency over power, making it appropriate for some but not all vehicles. Toyota Prius was the first of many hybrid vehicles to employ the modified Atkinson cycle, with quite a few other vehicle manufactures following suit.

Variable valve timing is used on many modern engines to produce increased torque over a broader rpm range than is possible with fixed timing, improve fuel economy and reduce emissions, depending on design intent. There are two general ways to alter valve operation; cam phasing and cam shifting. Phasing typically employs a hydraulic hub/sprocket that allows the camshaft(s) to be advanced or retarded relative to the chain or belt driven sprocket, allowing continuous and variable valve timing adjustment. Engine oil pressure is carefully metered to the actuator within the sprocket via a control solenoid, ordered up by the engine management system, based on cam position and other inputs.

Cam shifting is a term describing systems employing additional or unusual camshaft lobes, selectable rocker arms and other clever tricks to allow variable valve lift, timing and duration. At a predetermined speed/load the system shifts to a differing cam profile. Techie engines may employ both cam phasing and shifting. Using the correct oil and changing it regularly is important as these systems contain precise parts protected by easily clogged filtering screens.

Q: I have a 2013 Cadillac XTS with pure nitrogen in the tires. I have 25,000 miles on the car and had to add air four times since I’ve had the car (two years). I thought nitrogen use was to eliminate the need to add air?

I live in Florida, I filled the pressure to 35 psi and drove north. It was 0 degrees and my pressure went to 28 psi. I could not find pure nitrogen air. When I arrived in Florida, the pressure was back to 35 psi.

— Rich

A: Pure nitrogen is dryer and leaks less than regular air but will still sneak out of a tire over time. The large temperature swing you encountered caused the difference in tire pressure — they should be checked cold and set for use in each climate. I don’t find the minimal performance benefits of nitrogen to be worth the cost and sporadic availability for refilling. Tire inflation pressure should be checked monthly, regardless of what’s used.