Abstract

While developing an internal combustion four stroke cycle motorcycle engine for closed circuit road racing use, it was brought to the author's attention that a critical aspect of cylinder head design optimization with an eye toward maximum airflow for a given valve size is the correct selection of the port throat minor diameter to valve seat minor diameter ratio. It was suggested by a well respected authority on this matter that while previous testing performed by other researchers on similar cylinder head designs would provide a strong basis from which to select initial specifications, empirical test results would likely show a different optimal ratio than calculations based on previous research would provide, and would depend on case specific aspects of engine design, most notably valve lift. This paper will illustrate the equipment, test procedures, results, and conclusions of research performed on one Honda CB350 motorcycle engine's cylinder head.

Test equipment

A flowbench was utilized to measure changes in airflow brought about by modification of port dimensions. The type used during this research operates by using the pressure drop of air flowing through a flat plate orifice to quantify the volume of air flowing through said orifice per unit of time. As airflow rate through a flat plate orifice is not a linear function of pressure drop, it is advantageous to use an orifice sized such that the test is performed in a range of airflow to orifice area where large changes in pressure drop indicate small changes in airflow, to more accurately ascertain airflow rate via a manometer sampling pressure immediately before and after the orifice. With testing being performed at various valve lifts, and therefore flow rates, more than one orifice size is necessary to maintain the pressure drop within an optimal range for the airflow rates present during the test series. The flowbench used for this research accomplishes this by providing a metering plate with five orifice sizes which can be brought into use by removing a series of rubber stoppers. Used in various combinations, the orifices are suitable for testing airflow up to and including 185 cubic feet per minute at a pressure drop of 10 inches of water, which is adequate for measuring airflow through the valve and port being researched.

The cylinder head being tested is fitted to a cylinder of the same size bore as is to be used in the engine, to accurately model the airflow path which will be present during engine operation. Similarly, a manifold and muffler system such as will be fitted to the engine during track use is affixed to the exit of the exhaust port. The cylinder is fastened over an opening to a plenum in the flowbench which will be pressurized to 10 inches of water during testing. It is crucial to avoid air leaks at the mating surfaces of the cylinder head to cylinder interface, and cylinder to flowbench plenum interface, as this would tend to introduce error into the flow measurements. Lending to this need, either a gasket or a thin layer of viscous material such as wheel bearing grease should be used between mating parts before securing them together with threaded fasteners or clamps.

With the cylinder head to be tested fitted to the flowbench as described, a fixture providing a means of opening the valve controlling flow to the port being tested and measuring said valve's lift is fitted atop the head. This is most easily accomplished by drilling and tapping a hole in the fixture such that a threaded fastener is above and in line with the valve stem. The fastener can then be tightened against the valve's tip, overcoming valve spring closing pressure and opening the valve. A dial indicator reading off the upper valve spring retainer, which moves in concert with the valve, suitably supplies the test operator with valve opening measurements to within 0.001". The fastener and dial indicator should have a range of movement sufficient to fully open and measure the valve without changing or readjusting the fixture, to speed testing and maintain accuracy.

Test procedure

Before any testing can be performed, the flowbench must be calibrated for the day's air conditions. This is accomplished by fitting a plate with two orifices of known flow rates at 10" of water pressure drop to the plenum opening. Both orifices are circular in shape. One has a diameter of 0.312", and flows 4.5 CFM @ 10" H2O and standard atmospheric conditions of 0 degrees C and 760mm Hg. This orifice is used to check for leaks within the unit, as readings for such a small rate of airflow would be greatly skewed by even a small leak. The larger orifice is 1.875" in diameter, and flows 148.7 CFM @ 10" H2O and standard atmospheric conditions. With both the large and small orifices open, the flowbench should read as flowing 153.2 CFM @ 10" H2O under standard atmospheric conditions, when powered by 120 volts. This test confirms that the blower motor is performing to specification. Should this not be the case, a simple correction factor of 153.2/f can be calculated, where f is flow in CFM. This will be applied during calculation of raw data.

After removing the calibrated test orifice plate from the flowbench's plenum opening, the cylinder and head assembly is installed as previously described, with exhaust manifold and muffler. The dial indicator reading valve opening is set to zero with the valve closed. With all but the smallest flow orifice blocked, the flowbench is turned on to detect, and measure, any leakage that may be present through the valve seats or gaskets. If any is found, and cannot be eliminated, its rate is recorded for later use.

After removing all the rubber stoppers from the flow orifices and opening the valve to the lift being tested, the pressure in the plenum is adjusted to 10" H2O, as determined via a manometer comparing pressure between the plenum and ambient pressure. The rubber stoppers are then progressively installed in the flow orifices until the manometer indicating pressure drop across the flow plate is operating in a range sufficient to provide accurate determination of flow rate. Using a progressively marked scale along the manometer, percentage of maximum flow rate is read. If the orifices used are known to flow 100 CFM @ 10" H2O, and the manometer reading indicates 85.7%, the cylinder head is flowing 85.7 CFM at that valve lift, uncorrected for temperature. As the temperature of air flowing through the flowbench is raised by cooling the blower motor (a rather inconvenient design idiosyncracy), its density decreases. Recording air temperature within the flowbench before and after the motor is mandatory so that its decreasing density can be accounted for. A correction factor, based on temperature rise, is employed during calculation of corrected flow rate.

Correcting raw data to standard

So that tests performed under varying conditions can be compared, raw flow numbers must be corrected to a standard. This is accomplished through a series of calculations that, while simple, are numerous, as shown in the attached sample test data sheet. This introduces unwanted opportunity for error. To minimize this, a spreadsheet program was written in Microsoft Excel to performs these tasks, leaving room for error only in flow bench operation and data entry. Two samples from the Excel program are also attached. 'A' shows the formulae throughout the spreadsheet, 'B' gives data from a single test cycle.

Cylinder head and exhaust port

Port throat minor diameter over valve seat minor diameter and its effect on airflow for a given valve size, and therefore valve seat minor diameter, being the subject of this research, it was important to be able to accurately manufacture various port throat minor diameters while keeping other aspects of the airflow path constant. This was accomplished by using a readily available two part epoxy to build up the internal surfaces of the exhaust port, then cutting it to shape as necessary. Though this substance, Devcon Aluminum Epoxy, when cured exhibits ease of shaping and dimensional stability suitable for use under the conditions experienced during flow testing, it would be unsuitable for use during engine operation as it would quickly disintegrate under the temperature conditions experienced in an exhaust port of a running engine. Once an ideal port configuration has been determined using epoxy, it should be measured and recorded for reproduction after employing a skilled welder to build the internal surfaces with metal. The port throat minor diameter was established using a variable diameter cutting tool powered by a common hand held drill and piloted in the valve guide to keep the port concentric with the valve seat. Past work conducted by other researchers had suggested a port minor diameter to valve seat minor diameter ratio of 0.85 near optimal. In the interest of developing a wide ranging set of data from which to draw conclusions, ratios from 0.74 to 0.95 were tested here. As the valve size used during testing stayed constant at 30.0mm, and valve seat angle and width also stayed constant at 45 degrees and 1.0mm respectively, a valve seat minor diameter of 28.5 was used throughout the testing. Port throat minor diameters from 21.0mm to 27.0mm in 1.0mm increments were tested. An 8 degree included angle of divergence from the port minor diameter was retained by using a cone with an 8 degree vertex as a model from which to draw length to width measurements, which were applied approximately along the port's centerline by using a hand held die grinder and various rotary files to shape the epoxy as measurements with dividers and vernier calipers dictated. From the valve seat minor diameter, the die grinder and rotary files were again employed to cut a smooth radius connecting the 45 degree seat to a short straight cylindrical section at the minor diameter of the port. The resulting shape could accurately be termed a venturi.

Though camshaft profiles currently available from Honda and various aftermarket suppliers use a maximum of 0.350" exhaust valve lift, tests were performed in 0.050" increments up to 0.400", again in the interest of forming a wide ranging database from which to draw conclusions.

Test results

While examining data from airflow tests performed with the seven aforementioned d/D ratios, the following observations were made

1. Flow rate at lower valve lifts increased as d/D decreased.
2. Flow rate at higher valve lifts increased as d/D increased.
3. Optimum d/D ratio is different for each valve lift.
4. As d/D was increased past that which produced the best flow rate for a given valve lift, flow became audibly turbulent. Turbulence was also noted in fluctuating manometer readings.
5. Flow rate dropped more quickly for a given valve lift as d/D was increased beyond optimum than decreased from.

1. Changing d/D ratio has a large effect on airflow behavior.
2. Turbulence is more harmful to flow rate than reducing flow area is, and should be avoided when maximum flow rate over the range of valve lifts is desired.
3. Valve lift, and therefore camshaft design, must be taken into consideration when choosing d/D ratio.
4. Further testing, to determine what, if any, favor should be placed to flow rate at higher or lower valve lifts may be beneficial to engine power output.
5. Further testing to determine what, if any, effect changing d/D ratio has on intake port flow may be beneficial to engine power output.

References

Society of Automotive Engineers research papers:

Research and Development of High-Speed, High Performance, Small Displacement Honda Engines

S. Yagi, A. Ishizuya, I. Fujii

Air Flow through Poppet Inlet Valves - Analysis of Static and Dynamic Flow Coefficients

E. Watanabe, I. Fukutani

Design Refinement of Induction and Exhaust Systems Using Steady-State Flow Bench Techniques

G. Leydorf, R. Minty, M. Fingeroot

An Analysis of the Volumetric Efficiency Characteristics of 4-Stroke Cycle Engines Using the Mean Inlet Mach Number Mim

E. Watanabe, I. Fukutani

A Study of Gas Exchange Process Simulation of an Automotive Multi-Cylinder Internal Combustion Engine

Masaaki Takizawa, Tatsuo Uno, Toshiaki Oue, Tadayoshi Yura

Books

Internal Combustion Engine Fundamentals

John. B. Heywood

The Internal Combustion Engine in Theory and Practice

Charles Fayette Taylor

The High-Speed Internal Combustion Engine

Sir Harry Ricardo

Race Car Aerodynamics

Joseph Katz

Chevy Engine Guide

Smokey Yunick

Legend

d = port minor diameter

D = valve seat minor diameter

CFM = Cubic feet per minute