How does a late intake valve closing engine work?

By Craig Van Batenburg

The reason hybrids cars are here has everything to do with the internal combustion engine (ICE). Well, not the ICE itself, but what we put in the tank. If we burned a carbon-neutral liquid fuel made with a carbon-based product that grows naturally on the earth’s surface, and the energy used to produce, store and transport it was all renewable, then the ICE would be something we could live with. But that is not possible.

In terms of light-duty transportation, the hybrid electrical vehicle (HEV) — using an ICE that burned less fuel but still had the needed power — was the answer a couple decades ago, and that included “light-duty diesel” HEVs.

Before we look at how the gas engine in a hybrid electrical vehicle operates, a little history.

James Atkinson invented a new type of gasoline engine in 1887, commonly known as an “Atkinson Cycle.” It used valves, a camshaft, and a connecting rod that changed lengths to produce four piston strokes for every one revolution of the crankshaft. If you check online for this by searching Atkinson Cycle, you can watch the piston move at a different speed at each stroke. The intake and compression strokes were significantly shorter than the expansion and exhaust strokes.

These odd engines were produced and sold for several years by the British Engine Company. Atkinson also licensed production to other manufacturers, but the type of engine never caught on. If an Atkinson Cycle engine were to be used in a modern car, the crankshaft would not survive the repeated RPM changes and stress of shifting. Today, a real Atkinson Cycle engine exists, produced by Honda but only for a generator they call “Free Watt.” 

A “late intake valve closing” (LIVC) is the valve train and induction system that mimics, to some degree, what the Atkinson system was doing. LIVC is used in many, but not all, hybrid and plug-in hybrid models. LIVC is also used in conventional cars and trucks as well, but only recently.

When Toyota was creating the Prius in the early 1990s, they had one big advantage that allows them to develop a new concept based on the old Atkinson Cycle engine. They designed a powerful electric motor inside the transmission that allowed the use of a low-torque ICE with a high-torque electric motor, both powering the wheels. This was the first modern hybrid built for mass production. It was sold in Japan in December 1997. The Prius came to America in the summer of 2000.

LIVC is simple in operation. It is always a twin cam engine as it needs to vary the intake valve(s) timing. When used in a Toyota/Lexus vehicle, the timing of the opening and closing of the intake valves is controlled by the Toyota designed VVT-i system.

During the compression stroke, the intake valve is held open and the compression does not happen until the intake valve closes. The mixture is pushed back into the intake manifold, so you will note that the intake manifold has a chamber, or bellows, cast into it, allowing a space for the mixture to collect. At some point in the compression stroke, the intake valve will eventually close, and then the mixture will start to compress. During this compression stroke, some of the mixture moves into the intake manifold to reduce pumping losses. Pumping losses represent the power lost when the engine rotates during periods of high vacuum. LIVC changes the volume, or displacement, of the cylinder and also supplies a charge of fuel and air for the next cylinder in the firing order.

These engines have a high compression ratio number but operate at a low compression ratio. They perform best with low-octane fuel, something that is not characteristic of high-compression engines. The LIVC engine allows for a more efficient operation, but sacrifices were made in the total output of the engine. The ICE runs with normal displacement when the intake valves close earlier. This action provides more power output. Because the VVT-i system responds to operating conditions, the displacement of the engine changes accordingly. The VVT-i system is managed by the powertrain control module (PCM), and with that control, the intake valve(s) can change quickly (within a range of 40 degrees or more), depending on the model. The PCM adjusts valve timing according to engine speed, intake air volume, throttle position, load, and water temperature. In response to these inputs, the PCM sends commands to the camshaft timing oil control valve (OCV).

The VVT-i controller is located at the end of the intake camshaft. The PCM controls the oil pressure sent to the controller. A change in oil pressure changes the position of the camshaft and the timing of the valves. The camshaft timing OCV is duty cycled by the PCM to advance or retard intake valve timing.

As mentioned before, a powerful high-torque electric motor more than makes up for the loss of power in a LIVC engine. When you step hard on the “go pedal” (it is no longer just a gas pedal), the PCM senses the need for more power, and both the ICE and one or more electric motors are added to the total driveline output. A small ICE can act like a much larger one without the need for all that power all the time. This is one of many ways a hybrid can greatly improve the fuel economy and still perform well.

As stated above, the big change in the ICE is the valve train. The camshaft lobe is much more rounded. On most hybrids, cam phasing of the intake camshaft is common. The timing of when the intake valve closes determines the effective compression. Under many driving conditions, a low power output is desirable. If you have more power than needed, the consumption of fuel is higher with no gain. An aerodynamic vehicle with low weight and low friction-producing components will get you further on the same about of liquid fuel. There is more than just LIVC that can contribute to a more fuel-efficient combustion process, but if the ICE can work less, the fuel consumption will be better. The camshaft OCV works within the VVT-i system and can advance, retard, or hold the cam timing steady. The intake camshaft can rotate in response to oil pressure. To advance the valve timing, you add oil pressure to the timing advance side vane chamber. By moving the OCV, oil pressure is applied to the timing retard side vane chamber, and the timing is retarded. The PCM constantly calculates what position the intake camshaft needs to be in, and adjusts the oil pressure accordingly.

In addition to controlling the VVT-i system, the vehicle’s PCM also controls the fuel injection timing and the ignition system. Inputs from numerous sensors are used to optimize the ICE in order to provide maximum fuel economy.

There is no cable between the accelerator pedal and the throttle plate, as the continuously variable transmission requires precise control of the planet carrier inside the planetary gear set. If the throttle on a Toyota or Ford hybrid cannot be perfectly controlled, serious transmission damage will result. The PCM calculates the proper throttle opening and sends, via a CAN bus, the signals needed to open and close the throttle plate.

An important note: Always check and follow the service information for hybrid vehicles, even for something as “simple” as an oil change. Make sure you use the correct weight, type and amount; if it says to use 3.5 liters, remember that is not four quarts. If you overfill it, the ICE may not start a few weeks later. Precision, even in an oil change, is required for a hybrid to stay running.  •

Craig Van Batenburg is a former repair shop owner who is the CEO of Automotive Career Development Center (www.fixhybrid.com), which offers training and consulting related to electric and hybrid vehicles. He can be reached at Craig@fixhybrid.com.