specification of valve timing in spark ignition engines. The effect of .. These systems change the timing of the camshaft in relation to the crankshaft in order to. Today's topic: Variable Valve Timing. uses these sensors to monitor the relationship between the piston's location and the valves' positions. In a piston engine, the valve timing is the precise timing of the opening and closing of the valves. In an internal combustion engine those are usually poppet.
I mentioned earlier that advancing the cam will increase cylinder pressure. It will to a point. When the cam is advanced, the intake valve will open earlier during the exhaust stroke and the exhaust valve will shut earlier during the intake stroke. If the cam is advanced too far, reversion will occur and the exhaust gasses will not be adequately scavenged.
When the cam is retarded, cylinder pressure will be reduced but the scavenging process is increased. If you are experiencing pre-detonation, retarding the cam will help. It also has a tendency to move peak hp to a higher rpm. Again, care should be taken when changing cam timing.
Another consideration when playing with cam timing is piston to valve clearance. When you change the valve events timingthe clearances will change and should be checked.
Since we talking degrees, I might as well cover duration.
Duration is the amount of time that the valve is open in relation to crankshaft rotation. It is expressed in crankshaft degrees. If we have a cam with a duration of degrees, the valve will be open for degrees of crankshaft rotation. There are two methods used to describe duration.
Seat-to-seat or Advertised duration and at. The advertised duration is the measurement from the very beginning to the very end of the lobe ramps. It is difficult to get an accurate measurement using advertised duration.
Valve timing - Wikipedia
Theoretically, you should be able to find zero lift of the lobe ramps, but it is harder than it sounds. To simplify this method, cam grinders pick an arbitrary number unique to themselves. It could be anywhere from. Because cam grinders wont get together and give us consistent advertised duration lift points, they came up with a standardized method of. When the lobe is at. Cubic inch displacement, cylinder head characteristics, EFI, NOS, aspiration, compression, drive train, vehicle application and weight, desired peak power, desired engine operating rpm…….
LSA for a performance ground cam is typically between degrees. Sometimes even less than is ground for stroker engines. When duration is increased and LSA is constant, the valve overlap is increased.
When overlap is increased, vacuum is lower, cylinder pressure is reduced and reversion is increased. These are all undesirable traits for low end and midrange torque. You need cylinder pressure and vacuum for low end torque. Unfortunately, we cant have our cake and eat it too.
As piston speed is increased, the time that the cylinder can adequately fill and evacuate is drastically reduced. The only way to do this, is to increase duration and lift.
A very aggressive profile is also hard on the entire valve train and camshaft. Lift is the total height of the lobe.
VALVE TIMING AND ENGINE FIRING ORDER
It is a measurement that is described in inches. A lobe lift of. To get the total valve lift, we simply multiply the lobe lift by the rocker arm ratio. If we used rocker arms with a 1.
When looking at cam profiles, the lift listed is typically total valve lift using 1. If you want to know what it would be with 1. If you were to cut the lobe in half, both halves would be identical to each other. Depending on the grind, one ramp will be more aggressive than the other.
Cam grinders have found that the speed in which the valve opens and closes can greatly affect performance.
Typically, the closing ramp will not be as aggressive as the opening ramp on asymmetrical grinds. This will prevent the valve from bouncing off the valve seat when closing. The ramp provides a gentle transition from base circle to the flank. Ramps were first used for mechanical lifters that ran with a lot of lash.
As the cam rotates and the lifter hits the flank, the lash it taken up immediately causing a shock to the lobe and a noticeable tap when the rocker arm hits the valve stem tip. The ramp will allow the lifter to ride up on the lobe flank gently. As the lifter is traveling down the closing side of the lobe, another ramp is used to have the same effect on the lifter prior to making the transition from flank to the base circle.
When a hydraulic lifter makes the transition from the base circle to the flank, the initial shock will compress the spring in the lifter affecting total valve lift and duration. The opening and closing ramps reduce these initial and exiting shocks. Not all cams are ground with transition ramps and even fewer have closing ramps at all.
To aid the engine to effectively evacuate the exhaust gasses, dual pattern cams are used. A dual pattern cam will have a different lift and duration between the intake and exhaust lobes. A little more duration and lift on the exhaust lobe will give the engine more time to expel the exhaust gasses.
I have a tendency to ramble when I get talking about hottrodding. The main control loop shown in Figure 8 consists of a series of sequentially executed routines.
Each of these events occurs at a specific time. The software again used TMR0 to measure the time since TDC and to determine when this time catches the appropriate speed dependent times from the look-up tables. These two values come from the table look-ups in the speed calculation routine. It then saves the current value of TMR0 so that it can do the roll over check next time this first loop is executed. The software then checks to see if the rollcnt matches the exhaust valve open count EVOCnt.
The time for the end of the 2. In the personal computer method electrical signals are transferred to the computer in the form of binary numbers.
Contents of the input port of the digital card are thus brought into the computer memory. The software used to show the valve timing and valve displacement with crank angle degree manipulates input data.
The computer software consists of two measurement programs. First program is to measure the response of the model engine system start of valve opening, duration, closing, and valve lift with the crank angle degree. When that program started the software initialize to measure the engine speed by measuring the time each revolution of the crankshaft takes.
The valve open, close and lift are determined by using the analog signals volt signal from the linear position sensor and convert it to a position in mm. The software is adjusted to operate for a specific period of time namely one second. At the end of each second it sends the results to the Microsoft excel file.
After that the relation between the crank angle degrees and valve position can be monitored. The Lab View computer software, as shown in Figure 9, is used for controlling the model engine valve timing. The software starts if it detects the engine TDC; hence it starts the main control loop. The main control loop for computer control consists of series of sequentially executed routines. The normal sequence is to detect TDC, open and close each valve at crank positions determined by the speed calculation routine and look-up table values.
The intake or exhaust valve will open or close when the encoder sensor approach the proposed crank angle as given in the look-up table corresponding to the speed calculated by the speed calculation routine.
It is worth noting that the engine is put under control of the microcontroller or computer software so that the engine is always maintained very close to a certain desired valve timing corresponding to the best volumetric efficiency regardless of speed changes. Continuous sampling of engine speed and continuous correction of the valve timing do this.
The control loop of the microcontroller or computer software is mainly composed of input instructions; digital output instructions look-up table and control instructions.
The input instructions are sent by speed measurement port to transfer the measured speed value to the microcontroller or computer memory. The microcontroller or computer generates digital output instructions. For the required pulse width for the solenoids, the output signals are transferred to both solenoid actuators.
The new concept for self-regulation of optimum valve timing corresponding to the best volumetric efficiency was carried out. The new concept is done by the way of creating a look-up table of the optimum valve timing against speed. A look-up table is a list of related values stored in the memory unit of a microcontroller or computer; this table relates the output solenoid signal settings given by the microcontroller or computer to the input signals received from a given sensors.
Results and Discussions From the bode diagram shown in Figure10 bandwidth of the solenoid can be determined as follow: So the Bandwidth is equal 30 HZ, where bandwidth is the solenoid operating range of frequency. This result means that the solenoid actuator can be operated carefully from zero to 30 Hz frequencies, i. Then it will be operated from to rpm with less efficiency less valve lift. Therefore, a better valve timing can be predicated at each engine speed.
Table 2 shows the sample of recorded valve timings and duration that fed to the microcontroller and computer-based controls. It indicates that while the engine speed increased the duration of valve opening must be increased that is not offered by conventional cam engines. Figures show the variation of timing and duration of valve at different speeds with microcontroller control, Figures show that variation when using the computer-aided control. These figures show an accurate achievement of the look-up table values of the optimum valve timing covering completely the operation speed ranges.
The slope of valve lift to the crank angle decreases as the engine speed and, consequently, the valve opening duration increases. A steep decay of the valve closing velocity is noticed and the rate increases at higher speeds. Figures show a proposed and measuring valve response at various engine speeds. It is possible to notice that the electromagnetic control system is extremely sensitive to its microcontroller and computer-aided control program which alters the required optimum valve timings, and this will help to obtain an accurate valve timing for any type of engine by calculating the optimum valve timing to achieve the corresponding improved engine torque values at different engine speeds [ 23 - 26 ].
At valve lift When the induced coil is driven with a constant current, the steep decay of the velocity shows the hard impact with about 0. Consequently, a control system should fulfill the following requirements: The full valve lifting at most of duration that cause an improvement in volumetric efficiency related to increase the average valve lift and to reduce the average gas velocities.
Therefore, the flow resistance in the intake and exhaust ports is reduced. At valve falling or closing In this case, when the volt drops and the induced coil is released, the stored energy in the coil spring will be relieved and a steep decay of the velocity shows the hard impact with about 0. Consequently, a damper system must be used to avoid this problem.
Conclusions A continuously electromagnetic variable valve timing system is developed and implemented to meet the requirements of a higher volumetric efficiency and torque under all ranges of engine running conditions. The major conclusions are as follows: An electromagnetic camless valve train is developed for a camless model engine. This development confirmed its functional ability to control the valve timing and its duration. The proposed system demonstrated a full flexibility and quick response in the variable timing and duration.