Two-Stroke Engine Technology
Two-stroke engines, with their small size and mechanical simplicity, provide unique benefits to engine designers, manufacturers, and users. Although they were gradually abandoned for automotive use during the 1960s because of seemingly insurmountable emissions problems, new technology such as cost-effective in-cylinder fuel injection offers promise for their resurgence.
Development of automotive two-stroke engines must address:
Southwest Research Institute (SwRI), with more than 40 years of engine, fuel, lubricant, and vehicle research experience, has developed analytical and experimental techniques to quantify and solve problems associated with automotive two-stroke engines.
Ring Motion Modeling and Measurement
A combination of computer analyses, real-time oil consumption calculations, and blowby measurements is used to determine the cause of excessive oil consumption, blowby, and ring wear.
Highly developed models predict piston ring force balances (ring friction, cylinder pressure, ring geometry, and weight). The models provide new understanding of piston ring motion characteristics and facilitate design optimization for minimum oil consumption and blowby.
Ring motion measurement techniques, with telemetry similar to that utilized in piston temperature measurement, are used to supplement analyses.
Piston Temperature Telemetry
SwRI developed a self-powered, telemetry-based device for measuring engine piston temperature. The key component is the power generator, which uses piston acceleration forces to move a steel slug through a magnetic field, generating alternating current. Power is rectified and filtered to supply transmitter and sensor circuitry. Capacitive sensors are multiplexed to an oscillator that transmits temperature data as an RF signal to an antenna in the engine sump.
The resulting package, which has been tested at temperatures to 600 degrees F and engine speeds in excess of 5,000 rpm, is small, lightweight, and non-intrusive.
Real-Time Oil Consumption Measurement
A specially blended oil containing thermally stabilized sulfur is used, along with a sulfur-free fuel, in a real-time oil consumption measurement technique developed at the Institute. Continuous measurement of sulfur dioxide in the exhaust allows calculation of the oil consumption rate, and special sampling procedures are used to differentiate burned and unburned fractions.
The sulfur tracer technique replaces time-consuming oil weighing procedures. An entire matrix of engine speeds and loads can be mapped in one day with the sulfur tracer technique.
Real-Time Wear Measurement
Internal combustion engine component wear is monitored in situ on a near real-time basis using surface layer activation (SLA) techniques. SLA involves activating a thin layer on the wear component, then installing the activated component in a test engine. Activity levels are relatively small when compared to classical tracer techniques. The gamma-ray activity level of the component is monitored using a conventional sodium-iodide scintillation detector. The reduction in activity concentration is corrected for the half-life decay of the selected isotope, then correlated with a calibrated wear activity profile to determine wear over a given time period.
Noise, vibration, and harshness (NVH) are significant issues facing engine developers. Idle and light load combustion roughness are important elements of NVH.
In a novel and robust design adaptable to production engines, an optical spark plug probe and a head gasket ionization probe are being combined to facilitate the study of early flame kernel development, growth, and completion of combustion.
Because cycle-to-cycle data are obtained, the technology is particularly well suited to characterize cold-start cycle variability, misfire, stall, and idle roughness.
The effects of spark energy, air motion, turbulence, and EGR on flame development are readily determined using this technology.
Fuel Spray Characterization
SwRI has the facilities and experience to characterize and develop injection equipment. Laser diffraction and phase-Doppler drop size and velocity measuring equipment, high speed video and cinematography, phase discrimination probes, and high pressure bombs are available for detailed investigations of global spray development, spatial distribution of drop size, liquid mass fractions, and degree of vaporization.
SwRI has expertise in the design and development of many types of fuel injection and fuel control systems (conventional fuels, alternative liquid fuels, and gaseous fuels, both with and without air-assisted injection).
External Scavenge Systems
A spherical air pump designed by Institute engineers displaces almost its entire internal volume with each revolution, making it very attractive as a scavenge pump for two-stroke engines. High internal volume utilization offers potential for a package one-third to one-half the size of conventional blowers.
The pump can be designed to optimize the trade-off between internal compression ratio and volumetric efficiency, depending on the air delivery strategy chosen for the engine at various operating conditions.
SwRI developed a multicylinder computer program to provide a means for comparing various two-stroke engines and optimizing design parameters. The program facilitates the process of investigating design variables such as port geometry and pipe length over specified ranges and determining their influences on key performance parameters, including power, fuel consumption, and mass of air throughflow. The program is easy to use and allows setup of nested parametric cases.
The SwRI program is used to simulate crankcase-scavenged or externally scavenged engines, and custom features can be added as needed. Dynamic pressure wave propagation is simulated in the intake and exhaust systems, which can be made up of complex networks of pipe segments and junctions.
This brochure was published in February 1994. For more information about two-stroke engine technology, contact Daniel Stewart, director, Combustion and Emissions Department, Engine, Emissions and Vehicle Research Division, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-3657, Fax (210) 522-2019.