Interesting facts about camshafts
NB: The diagrams may not appear correctly on the screen, but they will come out correctly on the printout, due to the small image file sizes we used to make these pages appear fast on your screen. Printing this information will take approx. 7 pages.
There is some material to be found about the engine timing mechanism of the internal combustion engines in college and university libraries. There are books about optimization, noise reduction, analysis etc. Literature about actual design and manufacturing of cam profiles following to up to date perception is hard to find, though. Especially nowadays, with energy saving, climate changes and environmental issues are hot topics, the gas change and charge cycle needs to be studied and discussed more deeply.
The camshaft as the controlling device of the engine timing mechanism can be seen as the "door guard" for the gas flow. The cam lobe curve itself is the crucial part defining and controlling the gas change.
Not too long ago, this was mainly discussed in performance tuning and racing circles. Much of the knowledge and practical experience was born - as so often - in these groups. Probably you also were already asking yourself questions like:
- How to find an useable cam shape, if there is no camshaft available
- Are really all valve failures caused by material defects
- Couldn't an unfit cam form be the reason
- How to avoid or minimize premature camshaft wear
- How to check and analyse the timing mechanism to ensure it is error free
- Can a given engine gain from a different cam
- Where in the valve train may specific problems arise
Following you may find some answers to these and similar questions, along with some thoughts on cam design and shaping.
Failing timing mechanisms may have a number of reasons. Problems may occur in all parts of the valve gear or in the form of the cam lobe itself, and they may be hard to locate. Just to give an idea of how tricky these design flaws may hide, we will point out two possible pitfalls of cam lobe design. These flaws may not just be introduced in the manufacturing process, they may already be made in the theoretical construction of the cam lobe.
One example for design flaws: The lift of a given cam lobe will be extended by 10% - a percental increase of the valve lifting curve. This is incorrect for several reasons, and will have disatrous results on the valve gear under stress. It may, however, be advertized as "computer designed", and such "computed" cams are sold as such. There is even a piece of software, which implements this as a standard "stretch" feature. There is also non-scientific literature which suggests this, but that may just be called manipulation but not mathematical calculation as we see it.
One example for manufacturing flaws: A camshaft is ground with some randomly selected lobe profile, which just happens to fit some design parameters as valve lift and timing. Doing so, the initially correctly calculated cam profile for a VW Golf GTi may end up in a Honda motorcycle engine. Another manufacturer may then copy this cam and use it in a Honda Accord. This will not lead to a satisfying result. It is most advisable to have the cam lobe computed individually for each engine. Variations as little as some hundreths of a millimeter (.0004") may triple the resulting force - this is why we need respective criteria to evaluate a cam lobe profile.
Evaluating a cam lobe profile
You probably already have put some thought on the evaluation and quality of several engine components and may make substantial estimations. You may measure the cylinder barrel's roughness and conicity, check the crank for roundness and offset. Testing the valves for leaks is relatively easy. The honing angle of a cylinder barrel can be determined, and the quality of the induction and exhaust ducts can be benchmarked on a flow bench. There are applicable criteria for every component.
But what about the camshaft?
First, we can measure the min and max diameter, and then calculate the max valve lift. What else? We can measure the valve lifting diagram and run some comparisons. Experienced engine buiders can even judge the components based on that data and draw useable and well considered conclusions. But - how do we get to the max? Are we already close, or are still noticeable improvements to be gained? What can be improved, and if, how much? It runs well as it is - sometimes.
Progressing only by trial and error is tedious, insecure and, above all, expensive.
A precise evaluation of camshaft and valve train
There is an engine to examine. How do we calculate the maximum permissable speed with a given valve train? How much increase can be gained, if we lighten the valve train by a certain amount? Which strength must the valve springs have just to ensure adhesion of the valve train mechanism to the cam lobe at maximum speed? Should we look for it by testing? This is tedious, and expensive. Even if the engine is running smoothly then, the springs may be unneccessarily stiff and cause unneccessary friction, wear and power loss. Are there ways to avoid such damage? Can we avoid pitting, excessive break in and valve breaks just by choosing the right cam lobe form?
The "Knock Soft" software provides mathematically founded answers to these questions.
We have developed a graphical method for the quick and exact evaluation of a given valve train which also identifies the prevailing forces. Another approach is to record the lifting curve step by step and then to calculate the relevant values by progressive differentiation and integration. This is more elaborate but allows for comparing existing cam lobes with those newly computed.
The first methods to answer the questions described above were documented in the Fifties by Dr. Bensinger and his mathematician Dipl. Ing. Kurz. After the - recommended - study of the relevant works the interested reader may realize that:
The mathematical approach is complex. Summarizing, the fundamental approach is:
The starting point is a geometric form, the cam lobe. The cam lobe consists of several parts of geometric functions. The "base circle" merges into the "top circle" which again merges into the "base circle". The transitition from one circle into the other is solved by a mathematical procedure using sinus functions. These considerations aim to avoid unwanted acceleration peaks, i.e. the momentary transgression of tolerable forces.
The bottom line is: The line of thought goes from an entity described by mathematic functions (the cam) towards a functional element (the valve)
Let's try to reconsider and rework the problem of defining the ideal cam shape. The unbiased approach may lead to completely new ideas by reversing the line of thought: Starting at the functional element (the valve) we work towards a mechanism which operates the functional element in the desired way.
Provided there is an ideal motion of the valve, it would be desireable to describe that ideal motion (by the valve lift curve), and create which mechanism ever necessary to eventually generate that motion. Should that mechanism include a cam, it is unreasonable to assume that a series of functions of analytic geometry stringed together should necessarily describe the form of a cam lobe that would eventually generate that ideal motion. Rather, the cam would be described by a freely formed path instead of analytical functions.
Our "Knock Soft" software calculates exactly that path.
Cam lobe and engine characteristics
In general, engine characteristics are described by power and torque diagrams and engine graphs. Practically, engine characteristics are perceived as "thrust" or "power" at different engine speeds and operating states. A desireable characteristic includes good bottom end power, good pull from low revs without flat spots, free and easy revving up to high speeds, acceleration "like a rubber band" - in other words, constant torque over a wide RPM range.
Valve lift curves tell a lot about the engine characteristic to anticpate. Graph Erh. 1.1 compares two lift curves. An engine which provides a satisfactory characteristic with the low lift curve will also have similar characteristics with a higher lift. It will not change its idling or have the dreaded flat spots. Maximum torque and power will be at approximately the same speeds. Both engines will react similarily to changes in components relevant for the overall tuning. E.g. a longer exhaust hose will lower the peak torque speed on both engines. It it important, though, that the change cycle has approximately the same timing (begin / end) as shown in the diagram.
At higher speeds, the higher lift curve will provide more power output, because the appropriate flow rate at the same cross section allows for higher engine speeds. The flow restriction at higher speeds is lowered. At low and medium speeds, emission levels will be worse, power output at low to medium speed will also be (slightly) reduced at full throttle because of reduced gas swirl. On the other hand, the valve train will receive higher stress at lower speeds, so that the tolerable values should be recalculated.
Comparison of conventional cams vs. "Knock Soft"
The following valve lift diagram compares two graphs. The lower lift graph was taken from a Honda V4 Superbike race engine with a "Honda Kit Cam" of the '90s. Those cams were sold by Honda at high prices and were already a - little - improvement over the stock cam.
An engine with these cams was running satisfactorily in general, but suffered from mysterious valve breaks during racing. The timing mechanism was analyzed, checked and found to be flawed and then recalculated. The aim was eliminating the valve breaks without sacrifice elsewhere. The maximum lift was left as it was, only duration was extended by 5 degrees as Honda's layout was pretty short for racing applications.
The gain in opening cross section is obvious. But - note that
Here is only gain without loss elsewhere!
With the recalculated cam, the spring strength can be reduced from 48 kp down to 29 kp (!). I.e. weaker springs can be installed, with all benefits, without sacrifices and with no risk for the timing mechanism. That means less friction, less wear, more power output, longer life cycle, less spring stress with the well known, hazardous consequences.
Furthermore, the original cam shows slow and creeping opening and closing. At 150 degrees, the original cam opened the valve for .01" already, while the computed cam keeps the valve still shut. A creeping opening means the valve has already left it's seat and is exposed to the hot gas while not having a noticeable lift. Without contact to the seat it can not dispense heat to the cooling seat but does not provide a noticeable free cross section either. The valve is marginally opened or closed and suffers from all thermo dynamic disatvantages, but does not have a positive flow dynamic impact. The valve is thermo dynamically open, but flow mechanically closed, which is undesireabe. The valve is passed by hot gas unneccesarily long without being able to dissipate heat to the cooling seat. That leads to a hot valve, hot combustion chamber surface, and in turn poor cylinder charge with all collateral disatvantages as lower fuel efficiency, higher emission and lower power output.
Practical racing experiences has confirmed the above calculation. The mysterious valve breaks did not occur anymore. This is quickly written and read, but imagine the success, the savings in cost and labor. We were pleased by the power increase, but this was more a welcome addition. Huge gains were possible with no sacrifices elsewhere.
But these experiences may not be generalized. As an example, let us look at a different case here. The engine has the same diplacement - 750cc - and the same application - Superbike racing. The following valve lift curve is taken from a Yamaha 750 YZF engine.
The comparison of both lift curves is completely different from the first example.
The diagramm above shows two curves. They look like one and a part of it just appears to bee fat. But there are really two. One ist the result of the "Knock Soft" calculation. The other one is the result of examination of the existing Yamaha cam.
Bear in mind that the results shown for the
existing Yamaha cam do not represent the values computed by Japanese engineers, but the computed values plus the sum of all errors from cam construction through production up to our measurement with inherent measuring and reading errors. But, despite the unfavorable preconditions, the diagram shows that with identical specifications as the starting point, we arrive at exactly the same results than our Japanese counterparts. In other words, given the same starting parameters,
"Knock Soft" would calculate exactly the same cam profile which is already
there. With mathematics and physics as the controlling factors, there is nothing to be done about it.
Is there still room for improvement?
The answer is: Yes
Difference between comparison #1 and #2
You certainly noticed: In our first example, we gained just by recalculating the cam lobe form. Further gains would be possible with further modifications.
In the second example, the cam lobe is already correct. There is no way to calculate it "more correctly", hence a simple recalculation without further modifications to the valve train would be futile. Only further modifications like lightening the valves, reducing the oscillating masses or improving the valve springs would open the door to further optimization.
Possible measures for engines as in example #2
Engines with correctly computed lift curves still leave room for optimization. Calculations for any kind of engine must fulfill certain predefined requirements. These may include life cycle, cost, availability, modular design etc. These definitions and settings are the "adjustment screws" which can be turned mathematically. If e.g. the specification "max cost for valve springs" is doubled, a potentially improved spring offers a new perspective for further calculation.
It is most important to have full control over the theoretical correlation to avoid unnecessary risk or losses, to minimize the number of test bench runs and to have a solid basis for these runs. For optimized engines, any change in parameters as valve weight (e.g. by lightening) or max engine speed (e.g. by using a different chip) will require a change in the cam profile to have the engin run with an optimized timing mechanism.
Thanks to Axel Griessmann for this translation from German to English. I hope he has done a good job. See my recommendet links if you want to know more about him,