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This paper presents an overview of techniques for measuring friction using bench tests and fired engines. The test methods discussed have been developed to provide efficient, yet realistic, assessments of new component designs, materials, and lubricants for in-cylinder and overall engine applications. A Cameron-Plint Friction and Wear Tester was modified to permit ring-in-piston-groove movement by the test specimen, and used to evaluate a number of cylinder bore coatings for friction and wear performance. In a second study, it was used to evaluate the energy conserving characteristics of several engine lubricant formulations. Results were consistent with engine and vehicle testing, and were correlated with measured fuel economy performance. The Instantaneous IMEP Method for measuring in-cylinder frictional forces was extended to higher engine speeds and to modern, low-friction engine designs.

In order to predict and understand the slippage between the cam and roller follower, the Cummins L-10 engine valve train was modeled. A mixed lubrication model was developed to investigate the friction and oil film thickness since the valve train is operated in very severe conditions. First, the roller follower pin friction was studied because it is a significant friction source for a valve train with a roller follower.(13) Using similarity analysis of the roller pin friction from the experimental data, the correlation between the friction coefficient and the Sommerfeld variable was obtained. Second, the interface friction between the cam and roller follower was studied. Oil film thickness and the asperity load were obtained based on the mixed lubrication model. Finally, the total friction force at each cam angle was calculated and roller slippage was predicted by comparing the tractive force and the friction force.

The valve train was instrumented to record the instantaneous roller speed, roller pin friction torque, pushrod forces, and cam speed. Results are presented for one exhaust valve of a motored Cummins L-10 engine. The instantaneous cam/roller friction force was determined from the instantaneous roller speed and the pin friction torque. The pushrod force and displacement were also measured. Friction work loss was determined for both cam and roller interface as well as the upper valve train which includes the valve pushrod, rocker arm, valve guide, and valve. Roller follower slippage on the cam was also determined. A kinematic analysis with the measured data provided the normal force and contact stress at cam/roller interface.(1) Finally, the valve train friction was found to be in the mixed lubrication regime.(2) Further efforts will address the theoretical analysis of valve train friction to predict roller slippage.

The distribution of fuel-air mixtures in many L-head engines is not homogeneous. If local mixture is too rich or lean, incomplete combustion occurs. This can play a major role in unburned hydrocarbon and carbon monoxide emissions. Fuel-air mixture distribution depends on in-cylinder swirl and turbulence and is directly related to intake manifold configuration, fuel delivery system design and combustion chamber shape. Understanding the spatial mixture distribution may help improve the design of these aforementioned components. Consequently, a more complete combustion process may result, and emissions reduced. A method that measures the emission of CH and C2 radicals via the use of an optical fiber bundle was used in this research to map the mixture uniformity in the combustion chamber. The intensity ratio (IC2/ICH) was correlated to the fuel-air equivalence ratio. The mixture distribution measured was then correlated with the hydrocarbon emission sequence.

A semi-empirical engine piston/ring assembly friction model based on the concept of the Stribeck diagram and similarity analysis is described. The model was constructed by forming non-dimensional parameters based on design and operating conditions. Friction data collected by the Fixed-Sleeve method described in [1]* at one condition, were used to correlate the coefficient of friction of the assembly and the other non-dimensional parameters. Then, using the instantaneous cylinder pressure as input together with measured and calculated design and operating parameters, reasonable assembly friction and fmep predictions were obtained for a variety of additional conditions, some of which could be compared with experimental values. Model inputs are component dimensions, ring tensions, piston skirt spring constant, piston skirt thermal expansion, engine temperatures, speed, load and oil viscosity.

A new bench tester for laboratory simulation of piston ring and cylinder wear has been developed. Tests are made using liner segments which bear against a reciprocating piston ring. Temperatures up to 550°C, and loads and speeds representative of the most severe top ring conditions may be imposed. A precision oil spray system delivers the desired quantity and quality of oil to the wear interface. The computer controlled simulator duplicates the desired test cycle, and displays and stores data on friction forces and friction coefficients as the test proceeds. In this paper results are presented from the simulator for production and prototype ring and liner combinations, including ceramic coatings for potential use in advanced diesel engines. The importance of the method of oil delivery on test repeatability is emphasized. Some comparisons with Cameron Plint bench tests and firing engine results are presented.

The piston and ring assembly friction of a lightweight piston with lower compression height has been compared to that of a production assembly. Additional weight was added to the lightweight piston to study the effect of that variable alone. The lightweight piston reduced friction, especially in motoring tests. Within the speed range tested (up to 1640 rpm) the friction reduction of the lightweight piston could not be attributed to the lower mass itself.

This paper describes the friction characteristics of a 1.8 Litre J-car piston and ring assembly as influenced by oil rings of conventional design, but of varying tensions. In addition, the piston-ring assembly friction characteristics are reported for a set of oil viscosities ranging from 2 to 20 cSt with and without a molybdenum friction modifier. Multigrade oil results are shown also. Finally comparisons are presented between changes in friction measured by the Instantaneous IMEP Method and those measured by the dynamometer for the engine as a whole. Our results show large differences in piston-ring assembly friction as oil ring tension was varied. However, these differences became moderate after the oil ring broke-in. Both high and low oil viscosities increased piston and ring assembly friction. The friction modifier was most effective with a mid-range viscosity and provided virtually no benefit at viscosity extremes.

Typically, engines with relatively few cylinders have required higher cranking speed to start in low temperature ambients. Of the several factors that contribute to cold startability, this study has focused on the instantaneous speed variation of the engine during cranking. This theoretical computer study revealed that engines slow substantially during compression and that the lengthening of compression time is exaggerated as the number of cylinders is reduced. It is hypothesized that long compression time creates excessive heat and blowby losses. In turn these produce low compression temperatures and pressures, hence greater difficulty in igniting the charge especially under cold cranking conditions were average engine speed is low, By matching the compression times of various engines designs, the relative average speeds required to start can be predicted with reasonable accuracy.

The Instantaneous IMEP method has been used to measure piston and ring assembly friction in a production Chevrolet 1.8 litre L-4 and a 5 litre V-8 engine modified for single-cylinder operation. Friction measurements are reported at different loads and speeds up to 1640 RPM under firing and motoring conditions with various oils and before and after break-in of the oil ring. Oils used were SAE viscosity grades 30, 50 and 30 with a friction modifier. Differences were found between motoring and firing friction, especially on the power and exhaust strokes. These differences diminished at higher speeds and lower loads where lubrication was more hydrodynamic. Differences in response to viscosity and friction modifier changes were noted between the two engines.

An experimental technique termed the Instantaneous IMEP Method has been developed to measure piston and ring assembly friction. The technique requires very accurate measurements of cylinder pressure, connecting rod force and calculation of inertial forces. Friction force is the difference of these forces in consideration of the slider-crank geometry. A grasshopper linkage has been used to transmit the connecting rod force signal measured by a strain gage bridge. Inertial forces have been calculated with the assumption of distributed connecting rod mass. The test engine was a Chevrolet 5 litre V-8, modified for single cylinder operation. Piston and ring assembly friction has been determined under motoring conditions with and without compression as well as firing. Friction measurements have been made with SAE 30 and 50 grade oils at different temperatures. Boundary friction has been observed especially near top and bottom dead centers.

Air motion in one cylinder of a Detroit Diesel 6V-92 two stroke diesel engine was studied under steady flow bench test conditions by a laser Doppler anemometer and an axisymmetric finite difference fluid dynamic model. The effects of four different exhaust opening geometries were explored. Measurements and calculations showed that the swirl induced by the 18 angled inlet ports produced non-uniform axial velocity profiles and large peaks in the mid-radius region (between cylinder center and wall). The exhaust opening geometry in the head of the cylinder influenced these axial velocity fields especially in the upper region of the cylinder. The study concluded that more uniform flow, which is favorable to the scavenging process, can be achieved by an exhaust opening located close to the cylinder periphery.

HEAT BALANCE STUDIES were conducted on a 1975 production 5.74ℓ V-8 and an experimental 3.69ℓ V-6 to determine sources of fuel economy differences when they were installed in a vehicle. Heat balance results, friction and fuel economy were explored during dynamometer tests simulating road load. Comparisons were made with and without certain emission control features. With comparable calibrations, the road load indicated efficiency was nearly the same for each engine as it would be installed in a vehicle with an appropriate axle ratio. The results suggest that engine friction and not combustion efficiency accounted for the major fuel economy difference.

Warmup of the Du Pont model V reactor during unchoked engine operation with air injection has been characterized by a nonreactive period, followed by a transition to an ignited condition. The early period is quenched by heat loss. The transition is gradual for hydrocarbons, but more abrupt for carbon monoxide. Model building for the warmup period is directed to the objective of developing a rapid computer simulation to predict light-off times and temperature histories for various reactor designs and operating conditions. Reactor gas temperature and chemical conversions are calculated as solutions for an ideal backmix reactor. Heat balances maintain a record of all reactor metal temperatures for the given configuration. Heat transfer by radiation, convection, and conduction is considered. The presence of a hot spot in the reactor has a strong effect on time to light-off. In addition to lowering the time, such an ignition source shows a great sensitivity to combustible concentration.

Cyclic combustion variations were studied in a single-cylinder CFR engine with a pancake combustion chamber. Variations in the combustion duration were shown to be related to mixture velocity and its variations through a simple model. This model postulates a critical flame radius increment in which mixture motion variations near the spark electrodes create the cyclic combustion variations. From experimental measurements, the critical flame radius increment in this engine was found to be about 0.4 in. The mixture motion measurements were made with a hot-wire anemometer in the engine motored without fuel. A new calibration and calculation technique was developed in order to obtain velocities from the anemometer output in the varying temperature and pressure environment of the engine. Tests were run at full and part load over the speed range of 500-1500 rpm.

A comprehensive analytical and experimental study of thermal reactors has been made. The findings have been incorporated into a computer model capable of simulating reactor performance both during warmup and steady-state operation. This paper treats the warmed-up case. Arbitrary reactor configurations may be explored in regard to the steady-state extent of oxidation of hydrocarbons (HC), CO, and H2 with perfect or imperfect mixing and the detrimental effects of heat losses. This experimental and analytical program focused on the Chevrolet 350 in3 engine-Du Pont model V reactor combination. Experimental and calculated results are presented and compared where possible. Analytical models for the Du Pont reactor were developed each of which treated the exhaust ports, core, and annulus differently.

Cylinder pressure variation is a fundamental and widespread combustion problem in spark-ignited engines. The basic factors causing this problem are variations both in the start and in the rate of combustion. These variations occur not only from cycle to cycle within each cylinder but may also show up as consistent differences between cylinders. Our test results indicate that the major cause of cyclic combustion rate variation is the mixture velocity differences that exist within the cylinder near the spark plug at the time of ignition. As yet we do not know how to reduce the cyclic mixture velocity variations and thus reduce the problem at its origin. However, it is possible to circumvent some effects of cyclic pressure variation by increasing the average combustion rate.