Let’s Clear the Air: Part Two The Evolution of the Hybrid Electric Vehicle August 2012
By Mark Schmidt, NAFTC
This article is part two in a series describing the evolution of the hybrid electric vehicle in this country. Part one can be foundhere. This article will cover unique characteristics of the Toyota Prius, including its Atkinson Cycle internal combustion engine (ICE). The Toyota Prius gasoline engine is equipped with special components that allow for its operation on the Atkinson Cycle, which complements the hybrid electric drive powertrain. This engine is an inline 4-cylinder, 1.5-liters, 16-valve Dual Over Head Camshafts (DOHC) engine that is designed to operate over a high-expansion ratio Atkinson Cycle. This engine technology is used in all three generations of Prius gasoline engines. Operation on the Atkinson Cycle is made possible by Toyota’s VVT-i (Variable Valve Timing-intelligent) system. This system is also used in conjunction with ETCS-i (Electric Throttle Control System-intelligent) and an EGR system employing a highly efficient EGR cooler. In addition, use of an electric water pump, power steering pump and AC compressor has eliminated the need for a V-ribbed belt. Thus, improved engine performance, quietness, fuel economy, and cleaner emissions have been achieved. I feel the need to make a statement before we continue. As we examine the workings of the Atkinson Cycle design, we will examine when the intakevalve closes and how this concept relates to reducing pumping losses. The key to understanding the Atkinson Cycle is to focus on what differentiates it from conventional engine valve technology, as only full hybrid technology is able to take full advantage of the Atkinson Cycle design. The valve timing specifications illustrated in Figure 1 are for the second generation Prius (2004-2009). The first generation Prius (2001-2003) works under the same concept; however the range of valve opening and closing is slightly different. Notice the range of possible times that the intake valves may close in relationship to crankshaft degrees of rotation: 61° after bottom dead center (ABDC) to 102° ABDC. Considering the fact that the piston is traveling upward during the compression stroke in the range listed above, a comparison may be made between non-Atkinson (conventional non-hybrid application) and a hybrid vehicle.
Figure 1 – Atkinson Cycle Valve Timing
In most non-hybrid engine powered vehicles, the intake valve is closed much sooner, or closer to bottom dead center (BDC) just after the fuel is injected. As a result, the air fuel mixture will begin being compressed as the piston is traveling upward, utilizing the long sweep of the compression stroke. What is different about an Atkinson engine is that the intake valve may be closed very late into the compression stroke 102° ABDC. The advantage of leaving the intake valve open as the piston travels upward is that the compression area now includes the intake manifold plus the combustion area. This enlarged pressurization area will reduce pumping losses, discussed below. Just before the intake valve closes, the fuel is injected. From this point forward, the air fuel ratio is compressed and the spark plug ignites the air fuel mixture as normal. Conventional engines have a slight advantage over Atkinson Cycle engines with respect to low end torque; however, the torque of electric motors in hybrid vehicle mates well with this operation. For non-hybrid vehicles, turbocharging or supercharging may be used in conjunction with variable valve timing and direct injection to increase engine torque and efficiency. The use of variable valve timing on both the intake and exhaust valve camshafts is becoming more common in many vehicles.What are pumping losses?What advantage is there to pumping air from the combustion chamber into the intake manifold? The answer is that positive pressure helps prevent pumping losses, which reduces wasted work. Under certain conditions, the throttle plate creates a negative pressure (vacuum) on the top of the pistons. As the pistons travel downward, resistance created by vacuum tries to slow their motion. That resistance is the negative pressure on the top of the pistons. The pressure pumped into the intake helps equalize the pressure so it is closer to the same pressure on both sides of the throttle plate, thus reducing pumping losses.Below in Figures 2 and 3, the components of the Atkinson Cycle valvetrain are displayed.
Figure 2 – Atkinson Cycle valve train. Credit: Toyota
Figure 3 – Atkinson Cycle camshafts. Credit: Toyota
In part three of this series, we will examine the changes between the first generation Prius and subsequent models.