Modern engines, both hybrid and non-hybrid, use Variable Valve Timing (VVT). VVT replaces the EGR valve by closing the exhaust valve early, leaving some exhaust in the cylinder for internal EGR. This eliminates the need to route “yucky stuff” back into the intake.
To improve fuel economy, especially in hybrids, an engine can be switched from the normal Otto cycle to the Atkinson cycle. In the Atkinson cycle, the intake valve is left open longer, pushing about half the air-fuel charge back into the intake manifold. This reduces power but drastically improves fuel economy by running on a partial charge. The lost power is compensated for by the hybrid electric powertrain, which assists with acceleration and smooths out the engine’s operation, particularly when cruising.
Atkinson cycle engines often require a way to trap hydrocarbon vapors because gasoline can pool in the large intake runner and plenum due to the cycle’s operation. An activated charcoal filter is used to trap these vapors when the car is off, preventing gas odors. This technology is becoming popular in non-hybrid vehicles as well (e.g., Toyota Tacoma) to improve fuel economy and emissions.
When diagnosing problems in hybrids, technicians must consider the Atkinson cycle, as the electric motor often compensates for the gas engine’s irregularities.
Electric motors in hybrids use a Resolver to determine the exact position of the rotor, which is essential for timing the three-phase power delivery. Every three-phase motor has a resolver. The resolver uses an excitation circuit (a constant signal) and sine/cosine circuits to sense the position of an eccentric washer on the rotor. This provides highly accurate position data (around 1040 positions per revolution) even when the motor is still. The computer uses this position and the desired direction (forward or reverse) to determine the firing order for the motor. A faulty resolver means a non-functional motor.
The power to the three-phase motor is controlled by a six-pack of IGBTs (Insulated-Gate Bipolar Transistors), which are essentially super-duty electronic switches. The computer uses these switches to deliver high current and voltage to the motor windings in the correct sequence (firing order) and speed, based on the input from the resolver. At least two IGBTs must be on at any time to create a circuit (voltage in and voltage out).
Finally, any three-phase motor can suffer from stray voltages causing bearing damage. Tools are available to measure stray voltage at the bearing shaft and analyze noise/vibration to detect bearing wear that is inaudible to the human ear.
