TOP 3 CONSIDERATIONS FOR BUILDING A VARIABLE VALVE TIMING TEST STAND Abstract Automotive companies are investing in variable valve timing (VVT) technology to create commercial internal combustion engines with optimized performance, improved fuel economy, and low emissions. In both the design and manufacturing phases, powerful, flexible VVT test machines are needed to accurately evaluate next-generation VVT engines for fast time-to-market. This white paper discusses the top three considerations and challenges for building truly versatile, high-performance VVT test stands that meet the demanding testing needs of the industry. Figure 1. Test machines for the next-generation VVT engines need to be versatile, highly configurable, and state of the art. The Fundamentals of Variable Valve Timing The basics of internal combustion engines comes down to the four-stroke combustion cycle for converting gasoline into motion. Typically a rotating camshaft is used to push the valves that control the flow of intake and exhaust gases into and out of the combustion chamber. The valve timing is the same, regardless of the engine speed. That means at every speed except for one, a fixed camshaft is not running at optimal performance. However, the faster an engine runs, the more air it requires. This can be allowed by opening the intake valves earlier and closing the exhaust valves later. Like the name suggests, variable valve timing seeks to vary the valve timing to operate efficiently at a wider range of speeds. This adjustment leads to better conversion of fuel to power and reduced emissions. Various methods of VVT are used in production today, such as cam phasing, cam changing, and cam oscillating. Cam phasing is currently the most commonly used technique in commercial VVT engines, due to its simple but fairly effective mechanism. For example, by slightly rotating the inlet camshaft by 30 degrees at high speeds, its phase angle is shifted such that the intake valve opens earlier. It also closes earlier, since phase shifting does not change the duration of valve opening. In order to shift both the intake and exhaust camshafts separately, double overhead camshaft phasing is required. Discrete cam phasing systems only offer two to three fixed phase positions, whereas the higher-performance cam phasing systems use continuous cam phasing to smoothly provide the most suitable phase angle at any engine speed. Figure 2. A double overhead camshaft valve train layout could take advantage of VVT on both the intake and exhaust camshafts. Consideration 1: Type of VVT Testing Given the right equipment and expertise, almost any component or parameter of an engine can be measured or controlled. Therefore, the type or types of VVT testing required will first need to be established. The three sections below include the major types of testing applicable to VVT systems. VVT Performance Testing Testing is commonly used to optimize the effectiveness of VVT technology in various operating conditions. Using different engine speeds and system oil pressures, the VVT system is evaluated for different parameters, such as response time when moving between two phase angles. By measuring the system’s response to the engine’s dynamics, resonance and harmonics, the effect on the system’s entire performance can be characterized. VVT Endurance Testing Endurance testing is commonly used to uncover potential points of failure and predict service life of products. These testers run the VVT system for long periods of time at varying speeds and phase angles to simulate long-term wear and tear. Applying higher-than-recommended oil temperatures or using unfiltered, dirty oil as lubrication, is one way accelerated life testing is used to intentionally increase the deterioration on the system. Cam Phase Testing In cam phasing VVT systems, an important area of test is the accuracy of the cam phaser unit. Does it precisely and unfailingly reach its assigned angular location for different engine load and speed requirements? These types of test stands can monitor the phase angle between the camshaft and crankshaft using the engine sensors or external encoders. Consideration 2: Subsystems of the VVT Test Stand Depending on the types of VVT testing that are needed, there may be several subsystems to the VVT test stand that need to be designed, built, and integrated with one another. Consider the following subsystems as you are planning out the requirements document for the entire project. Test Stand Controller This piece of the VVT test stand is ultimately the brains of test system and used to control the hydraulics and Buy vs. Build? drive subsystems mentioned below. Determining the sensor types and performance metrics needed for the At Wineman Technology, we highly recommend application influence the type of hardware and other utilizing commercial off-the- equipment that are used in the tester. shelf (COTS) equipment whenever possible. By The controller is responsible for the data acquisition and using COTS hardware, you control system, with key functionality such as: can: Save development time by working with ready- Collecting real-time, high-resolution angular made technology position data on cams and crank sensors for Build upon the comparison of “commanded” to “actual” cam expertise and location investment of engineering product Controlling and commanding engine speed, oil manufacturers temperature and pressure, and other key Receive support if a variables component doesn’t Acting as the engine control unit (ECU) in regards work or needs to commanding the VVT hydraulic valve or valves replacement The software and tools chosen to program the controller with this functionality must have the flexibility and customization needed to meet these stringent requirements. Figure 3. In a past project, Wineman Technology used INERTIA test cell software in combination with other COTS software, such as the NI Combustion Analysis System (NI-CAS) software plug-in and custom FPGA control to meet the stringent requirements of a VVT test system. A specific example task that the test stand controller is responsible for is the shifting of the camshaft phase. One technique involves using a belt-driven pulley, which is actuated by hydraulic valve gears. The hydraulic control valve is controlled by sending an appropriate pulse-width modulating (PWM) signal to a solenoid that modifies the valve orifice. In order to ensure that the phase shift has correctly occurred, trigger wheels that generate digital pulse trains can be attached to both the camshaft and crankshaft. Measuring the phase difference between the camshaft and crankshaft pulse trains thus can be used to calculate the current camshaft phase. The position control system would be responsible for sending out the appropriate PWM signal, measuring the digital pulse trains, and analyzing the data for reporting purposes. Hydraulics System The oil lubrication system is essential to making sure all the moving parts of the engine, such as the camshafts and crankshaft, move and function easily. Therefore, a hydraulic power unit can be used to act as the engine oil pump or to recirculate the oil in the engine oil sump. Variable oil volumes or pressures must be directed to the VVT component based on commands from the ECU and measurements of engine speed, temperature, load, and other variables. Controlling the oil temperature is important to ensure a properly functional engine under different conditions. Temperature control can be provided using a COTS temperature controller that heats the oil using a circulation heater and cools the oil with a shell and tube heat exchanger using plant water. Requirements – such as the types of oil that will run in the machine, the maximum and minimum temperature, the minimum temperature rise rate, and the minimum temperature decrease rate – will all affect the component selection process. When the hydraulic power unit is acting as the oil pump, flow measurements may be needed to measure how much oil is being consumed by the VVT system. To accomplish this, a gear-type positive displacement flow meter rated for 2 to 20 gpm can be used. The purpose of the flow measurements, flow ranges to be measured, and accuracy of those measurements will greatly affect the type and cost of flow meter required for the system. Drive System Finally, a system is needed to physically drive the engine so that it operates at certain speeds and loads for testing purposes. Common requirements for choosing the appropriate driving dynamometer include: Speeds at which the engine runs Duration of the test Torque application Optional: torque measurements (with the necessary torque range and accuracy) The motor typically interfaces with the engine using a constant velocity shaft. Therefore, using a universal mounting setup may be beneficial for testing different engine heads. Figure 4. A complete VVT tester may consist of subsystems such as the test stand controller, hydraulics system, and drive system. Consideration 3: Mechanical Configuration of the VVT Test Stand Since testing needs may vary from manufacturer to manufacturer, here are a couple of final considerations that will affect the overall configuration, architecture, and specifications document of the VVT tester. Extent of Engine Completion VVT systems are often developed in parallel with other components, and therefore testing may need to occur before the engine is fully designed or built. However, characterization of various subsystems in the VVT engine is possible with the aid of machinery such as dynamometers to replace and simulate the missing components. Determining the overall system architecture requires evaluating what is missing and what equipment may need to be substituted. In some cases, the whole casting of the engine may exist with most of the final parts, but without any pistons. Therefore, a dynamometer may be used to drive the crankshaft in order to test the cam phasers without any fuel running through the engine. In other cases, only the heads for the valve train and the VVT components are available, which means the entire bottom half of the engine is missing.