The decision not to use cam followers with hydraulic compensation, was taken early on during the design phase. Most automobile and some aircraft engines have hydraulic compensation as a standard feature. Many people have asked us why we didn't incorporate this feature. Most seem to think that hydraulic compensators can no longer be missed, "because owner's of the xxxxxx engine have to constantly adjust the tappet clearance". If the engine is well designed, there will be no need to continually readjust the tappet clearance. One of our prototype engines has already operated for more than 400 hours (June 2005). The tappet clearance was first set when the engine was assembled. After 2 hours of operation, 2 of the 8 tappets needed a small adjustment. During the following 400 hours, the tappet clearance was checked 3 times and each time no further adjustments were necessary.
The manual checking of tappet clearance is a simple task anyone can perform. Removing the engine cowling and rocker box covers is a matter of minutes. Even for those who are not familiar with the ULPower engine, a hex key, 13mm spanner, and a couple of feeler gauges are all that is necessary to make sure that the tappet clearance is within limits. "But you don't have to check the tappet clearance in engines using hydraulic compensators" some will say! The trouble is that hydraulic compensators can hide problems in the valve train !
We have communicated with one Rotax 912 owner who had to make an emergency landing because of sudden engine failure. The problem was diagnosed as premature wear on a cam lobe apparently caused by a fault in the hardening process. Unfortunately for this pilot, this is proof that hydraulic compensation can really disguise a problem which could have been noticed beforehand if tappet clearance was easily verifiable as is on the ULPower engine.
There are several interfaces in the valve train where wear occurs. The normal wear characteristics of the valve train components should be well known by the manufacturer, so their maintenance and TBO schedules will require inspection and or replacement of components subject to wear at safe intervals. However if abnormal wear is happening in an engine with hydraulic compensators, this problem will probably go completely unnoticed until such time that the compensation needed to make up for the wear becomes greater than the compensator can accommodate. Serious damage to valve train and cylinder head components could easily be the result because the problem has not been noticed in the early stages.
Another problem with hydraulic compensators is that they can be the cause of valves not seating properly. If for some reason a valve has not been able to seat correctly once because of some small hard foreign matter sticking to the valve seat or valve lip, the hydraulic compensator will adjust to the valve's new (not completely closed) seating position. So even if the foreign matter causing the improper valve seating is eventually dislodged through vibration, higher gas flow, or other normal factors, the valve will continue to stay partially open! This situation will quickly lead to a burnt or misformed valve. If the hydraulic compensator's internal check valve is partially worn, it may leak some of it's internal pressure when the engine is stopped for a longer period of time. This could allow the valve to reseat itself properly again, but chances are that the valve and seat have suffered some damage and proper seating is no longer possible....
Our basic philosophy is this. Checking valve clearance is a simple and easy task which can be performed by almost anyone with just a couple of simple tools. If tappet clearance is changing significantly and you need to adjust the tappets relatively often, then this is a sure sign of trouble. This is a warning sign you simply may not ignore. Take action before any serious damage occurs and you will probably save yourself a lot of money for a serious repair job that could have been avoided. Don't let the perceived "luxury" of hydraulic compensators hide valve train problems which should not go unnoticed.
One person has told us that he could hear if a valve was not seating properly! but we have serious doubts that even the best mechanics could pick up that fault without specialist equipment. If there is a mechanic out there who can detect incorrect valve seating in an air-cooled aircraft engine just by hearing, we would love to talk with him....