Intake valve actuators (IVAs) are components found in modern engines, especially in diesel applications such as Caterpillar ACERT engines, that frequently generate curiosity and discussion among mechanics and automotive enthusiasts. Are these actuators primarily engineered to improve engine efficiency, or is their main purpose emissions reduction? This is a central question when examining the function and effect of IVAs. Some mechanics have noted potential improvements in miles per gallon (MPG) when deactivating these systems, raising further questions about their actual role and ideal operation. Let’s explore the mechanics of intake valve actuators, investigating their intended function based on manufacturer data and practical observations.
To understand the function of intake valve actuators, it’s helpful to first consider the Miller Cycle, an engine design concept that has been around for a while and is utilized in various engines. The standard Otto cycle engine operates on four strokes, with the compression and power strokes being the most power-intensive. A significant portion of an engine’s internal power loss comes from the energy needed for the compression stroke. Systems that lessen this energy consumption can enhance overall efficiency.
In the Miller Cycle, the intake valve remains open for a longer duration compared to an Otto cycle engine. Effectively, the compression stroke is divided into two distinct phases: an initial phase with the intake valve open and a final phase after the intake valve closes. This two-stage intake stroke creates what’s known as the “fifth” stroke in the Miller Cycle. As the piston moves upward during what is traditionally the compression stroke, some of the air-fuel mixture is pushed back out through the still-open intake valve. While this loss of charge air might typically decrease power, the Miller Cycle compensates for this by employing a supercharger. Positive displacement superchargers, like Roots or screw-type, are preferred due to their ability to generate boost even at lower engine speeds, preventing low-rpm torque loss.
A crucial aspect of the Miller Cycle is that actual compression only begins after the piston has expelled this “extra” charge and the intake valve closes. This occurs roughly 20% to 30% into the compression stroke. In other words, compression mainly happens in the latter 70% to 80% of the compression stroke. In spark-ignition engines, the Miller Cycle offers an additional advantage. The intake air is initially compressed by the supercharger and subsequently cooled by an intercooler. This cooler intake charge, along with the reduced compression during the intake stroke, results in a lower final charge temperature compared to simply increasing piston compression. This allows for more advanced ignition timing before detonation occurs, further improving efficiency. Another benefit of the lower final charge temperature is reduced NOx emissions in diesel engines, a critical factor in large diesel engines used in ships and power plants. Efficiency is improved by increasing the compression ratio. However, in typical gasoline engines, the compression ratio is limited by the risk of self-ignition (detonation) of the compressed, hot air-fuel mixture. Due to the shortened compression stroke in a Miller Cycle engine, a higher overall cylinder pressure (from supercharger boost and mechanical compression) is achievable, leading to better efficiency. However, it’s worth noting that positive displacement superchargers consume power, typically 15% to 20% of the engine’s output, to compress the intake charge.
According to Caterpillar (CAT) documentation, an Intake Valve Actuator (IVA), also referred to as a Variable Valve Actuator (VVA), is a device that interacts with the intake rocker arm to hold the intake valve open in ACERT Cat engines. This action is intended to lower cylinder temperatures, which helps in reducing NOx emissions.
The IVA system is electronically managed and hydraulically activated, operating similarly to a Jake Brake system. Engines are equipped with six IVAs, one for each cylinder. These variable valve actuators regulate the closing of the intake valves and only become active once the engine oil reaches a specific temperature. Oil for the IVA system flows from the oil filter base to an oil rail outside the cylinder head. If the oil temperature is too low, a diverter valve in the oil rail remains open, allowing oil to drain back into the head. Once the oil temperature increases sufficiently, the diverter valve closes, pressurizing the oil rail and the IVA housings. The oil pressure in this rail is maintained at approximately 250 ± 50 kPa (36 ± 7 psi) higher than the rest of the lubrication system. Bleed holes in the housings exhaust excess pressurized oil.
The IVAs function by holding the intake valves open beyond their normal closing point dictated by the camshaft lobe. When the solenoid is energized while the intake valves are open, pressurized oil fills a cylinder within the actuator. This pressurized oil pushes down a piston. As the intake valve begins to close under normal camshaft action, the valve rocker arm contacts this piston, which then holds the intake valve open longer. To close the intake valve, the solenoid is de-energized, allowing oil to drain from the cylinder. The valve spring force pushes up on the rocker arm, which in turn pushes the piston back to its normal position, allowing the intake valve to close completely.
Contrary to some beliefs, the IVA system is not designed to recirculate exhaust gas back into the intake for re-combustion. Instead, by holding the intake valve open during the initial part of the compression stroke, the system effectively reduces the amount of air-fuel mixture compressed in the cylinder. Since compression generates heat, reducing the compression volume lowers the combustion chamber temperature. Lower combustion temperatures are known to reduce the formation of NOx emissions. Given that the turbocharger provides initial air compression and the cylinder itself further compresses the charge, the IVA, operating during the first 15% of the compression stroke, aligns with the Miller Cycle principle where effective compression mainly occurs in the later stages of the stroke.
Therefore, theoretically, IVAs should not cause a loss of engine compression or power. In fact, based on the Miller Cycle principles and CAT’s description, the system is designed to enhance efficiency and reduce emissions. The idea that IVAs might contribute to soot buildup in the intake system also seems unlikely, as they are dealing with fresh intake air, not recirculating exhaust gases.
However, practical experiences sometimes present a different picture. The original poster mentioned backing off the IVA system on a 2005 T600 with an ACERT engine during an overhead adjustment as an experiment. This action suggests a real-world scenario where mechanics are exploring the effects of deactivating or adjusting IVAs, possibly seeking to improve MPG or address other operational concerns. The observation of MPG gains when IVAs are deactivated raises questions about the balance between emissions control and fuel efficiency in real-world applications.
engine overhead
Further investigation and data collection, such as monitoring fuel consumption and engine performance with and without IVA intervention, would be valuable to fully understand the practical impacts of intake valve actuators. While the theory suggests efficiency gains and emissions reduction, real-world applications and modifications might reveal a more complex picture regarding the optimal use and adjustment of these systems.
In conclusion, intake valve actuators in engines like CAT ACERT models are designed to reduce cylinder temperatures by modifying the intake valve closing timing, primarily to lower NOx emissions. The system operates on principles similar to the Miller Cycle, aiming to enhance efficiency while addressing environmental regulations. However, anecdotal evidence and practical experiments suggest a need for further exploration to fully reconcile the theoretical benefits with real-world performance and fuel economy considerations.
