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EVIC Update #5 - Speed and Power Control without Throttling
February 25, 2001 - Additional comments added December 26, 2001
Traditional ThrottlingThrottling an engine reduces the air/fuel charge drawn into the engine on the power stroke. By restricting the intake the pressure in the cylinder and hence the charge is reduced. This simple method of reducing the speed and power output of a 4 stroke cycle I.C. engine is very effective and hence almost universally used. The down side of throttling is increased pumping losses and hence reduced overall engine efficiency.
All IC engines, 2stroke, 4 stroke or whatever have pumping losses. Nothing comes for free. The discussion that follows applies to 4 stroke engines. As the piston rises energy (power) is used to push the exhaust gases out of the cylinder. Then as the piston falls additional energy is used to pull the fresh charge into the cylinder. The sum of these two energies is the engine's pumping losses. The amount of energy required can be reduced with larger and more valves. This is one of the reasons for the growing popularity of 4 valves per cylinder. The exhaust gasses and the intake charge both have mass and hence inertia. As the engine's speed increases the power required to move the gasses increases. If the intake is throttled, the opening into the intake manifold made smaller, it becomes harder to get the charge through this smaller opening and into the cylinder. It takes more power and the pumping losses increase. I am not sure what the pumping losses are for a typical I.C. engine. Perhaps someone can answer this question. However I do know that the designers of automotive engines are going to great lengths to reduce these losses. My guess is that a 5% gain in efficiency is possible if we could find another way to "throttle" an engine. I believe this efficiency can be gained by using variable valve timing to control the engine speed and power.
Micro Computer Control Provides an Alternative
Electronically actuated valves, controlled by a micro computer, provides alternative ways of reducing the charge in the cylinder. As with any technology there are advantages and disadvantages.
Early Exhaust Valve Closing
Early exhaust valve closing traps some of the exhaust gasses in the cylinder thus reducing the room for the fresh charge. This increases the pumping losses because the cylinder pressure increases at the end of the exhaust stroke. Unless combined with late intake valve opening it can result in exhaust gasses being forced into the intake manifold and even out the carburetor intake when the intake valve is opened.
Late Exhaust Valve Closing
Late exhaust valve closing causes some of the exhaust gases to be drawn back into the cylinder after TDC thus reducing the room for the fresh charge. This should have less effect on the pumping losses than early exhaust valve closing. However some energy will be required to reverse the flow of the exhaust gasses. Because the exhaust gasses are hot, their density is reduced and therefore the energy required to reverse the flow is lower than that required to reverse the intake flow. The pumping loss can be compared to, but will be less than, the energy used in hit and miss operation where the exhaust valve is left open for a complete crankshaft revolution.
If the intake valve is opened at the normal time some fresh charge will be lost out the exhaust port thus decreasing fuel economy and increasing emissions. Saab has proposed a system that solves this problem by direct injection of the fuel into the cylinder. The injection can be timed to occur as or after the exhaust valve closes.
Late Intake Valve Opening
Late intake valve opening, on its own, reduces the charge in much the same way as throttling. The pumping losses will be greater than those encountered with throttling because of the greatly reduced cylinder pressure in the first part of the intake stroke. One risk is that the intake valve could be sucked open if the intake valve spring is weak.
Late Exhaust Valve Closing Combined with Late Intake Valve Opening
Combining late intake valve opening with late exhaust valve closing should be an effective strategy when using a carburetor. Some of the exhaust gases can be drawn back into the cylinder after TDC thus reducing the room for the fresh charge. Because the intake valve is opened late none of the fresh charge should be lost. The pumping losses should be lower than those encountered with throttling because there are no throttling restrictions in the intake manifold. The proper 14.7:1 air/fuel ratio can be maintained. A hotter spark will likely be required to fire the charge if it is greatly diluted. Saab has proposed a mixture that could have up to 70% inert exhaust gasses. This does require a hot long duration spark to fire.
Early Intake Valve Closing
Early intake valve closing reduces the charge in much the same way as throttling. The pumping losses should remain close to those encountered with throttling because of the reduced cylinder pressure in the last part of the intake stroke. The intake valve is less likely to be sucked open but may be slower to close if the valve spring is weak.
Late Intake Valve Closing
Late intake valve closing reduces the charge by pumping some of the new charge back out past the intake valve into the intake manifold. This pseudo Atkinson cycle effectively decreases the cylinder volume and hence the compression ratio. The pumping losses are reduced because there is no throttling action in the carburetor or intake manifold. However the reverse flow into the intake manifold may result in mixture loss from the carburetor intake. Making the intake manifold larger and closing the intake valve before the manifold pressure exceeds atmospheric can make this an effective means of control. However I have found this very difficult to do in tests with my EVIC engine.
Late Exhaust Valve Closing
Combined with Late Intake Valve Opening and Late Intake Valve Closing
Combining these three strategies should provide an effective means of control. As a result of late exhaust valve closing and late intake valve opening the fresh charge has been reduced, diluted with inert exhaust gasses. Carried to the extreme, as Saab have proposed, a very hot long duration spark is required. If the dilution is limited to somewhere in the 30 to 50% range, a normal spark should be satisfactory. When a further reduction in speed or power is desired, late intake valve closing can be used. As noted in the previous section this effectively decreases the cylinder volume and hence the compression ratio. A lower compression ratio means less deceleration on the compression stroke and should allow for a lower idle speed for a single cylinder engine. The desired 14.7:1 air/fuel ratio can be maintained.
The problem of mixture loss from the carburetor intake can be addressed by increasing the volume of the intake manifold. If, at the lowest desired speed, there is still a loss of mixture from the carburetor intake some mild throttling of the carburetor intake can be used. I don't believe that any throttling will be required. With no throttling there should be a very significant gain in engine efficiency from decreased pumping losses. Even with mild throttling the gains should be significant. (See comment under Late Intake Valve Closing.)
My initial tests have demonstrated that the EVIC can be controlled as proposed. Step changes in the valve timing were introduced by closing test switches.
Late intake valve closing tests were done in December. These tests demonstrated a reduction in engine speed when the intake valve was closed 2 and 3 milliseconds later than "normal". With 3 millisecond late timing the problem of mixture coming out of the carburetor intake was observed.
The Late Exhaust valve closing and late intake valve opening tests were done February 24th. One test that delayed these actions by 2 milliseconds reduced the speed from 4300 RPM to 3150 RPM.
A 7/8" long spacer was made to add volume to the intake manifold. After this was installed a combination of these tests led to a speed reduction to approximately 2500 RPM.
During the course of these tests a highest ever top speed of 7500 RPM was obtained with the spacer in the intake manifold and some small changes to the valve timing. This was an unplanned test and hence a pleasant surprise.
The next steps are to modify the micro computer controls to allow more sophisticated control and tests. This may require a new or a second processor since I am running out of code space.
Late Exhaust Valve Closing
Combined with Late Intake Valve Opening and Early Intake Valve Closing
This proved to be the most effective strategy and is the one now being used for the EVIC's "Electronic Throttle". The combination of late opening and early closing of the intake valve effectively reduces the amount of new charge. The early closing of the intake valve has little effect on the pumping losses if the valve is closed near or just after bottom centre. - Dave Bowes, December 26, 2001
This work has forced my experiments with double acting valves to the back burner.
Your comments and suggestions are most welcome.
Dave Bowes, EVIC Designer
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