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This Australian design project has had 20 years of development.


This Australian Sytech engine was purpose-designed as a ‘range extender’ for some electric vehicles and a turbocharged 1.5-litre version is scheduled to enter production with an electric vehicle maker in China in 2025. The Sytech engine uses a modified ‘Scotch Yoke’ principle.



We first heard of the Australian SyTech (sic) engine development in 2000, when it was introduced at the FISITA 2000 World Automotive Congress, held in Seoul, Korea. Being conceived as a hybrid vehicle powerplant, it boasted weight savings over conventional combustion engines, with inherent near-perfect balance that implied greatly reduced noise and vibration. 

The output torque was said to be more uniform under all normal operating conditions than with conventional engines and the small size of SyTech engines made them especially suitable for the more complex packaging requirements in a hybrid vehicle. 

More than 20 years after its Seoul debut the latest Sytech engine range has undergone significant development at ASF Technologies and a subtle name change.



The key to the engine compactness is its use of a Scotch Yoke that’s also known as a slotted-link mechanism. This design converts linear sliding motion into rotational motion. The piston is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part.

The location of the piston versus time is simple harmonic motion: a sine wave with constant amplitude and constant frequency, given a constant rotational speed.

The side rods of some steam locomotives have Scotch Yokes that permit vertical motion of intermediate driving axles, but modern Scott Yoke mechanisms are most commonly used in control valve actuators, in high-pressure oil and gas pipelines and in some hot air engines.



Early efforts



The earliest effort to employ a ScotchYoke in an internal combustion engine was the 1920s Bourke engine, that never made it into series production, despite many years of effort. The main claimed virtues of the design were that it had only two moving parts, was lightweight, had two power pulses per revolution and did not need oil mixed into the fuel.

The Bourke engine featured a horizontally opposed piston assembly, using two pistons that moved in the same direction at the same time, so that their operations were 180 degrees out of phase. 


The pistons were connected to a Scotch Yoke mechanism in place of the more usual crankshaft mechanism, thus the piston acceleration was perfectly sinusoidal. This caused the pistons to spend more time at top dead center than in conventional engines. The incoming charge was compressed in a chamber under the pistons, as in a conventional crankcase-charged two-stroke engine. A connecting-rod seal prevented the fuel from contaminating the bottom-end lubricating oil. 

Russell Bourke’s two-stroke engine suffered from rapid wear of the slot in the yoke, caused by sliding friction and high contact pressures, but the Sytech engine would seem to have overcome that issue, by its use of a larger, pressure-lubricated sliding block in the yoke. 



Another strengthening feature is the use of a single-piece yoke, connecting both pistons on what is virtually a single rod in the Sytech engine. The opposed pistons also do not need conventional gudgeon pins, because the pistons-to-rod connections are fixed. Force is applied directly in the line of travel of the assembly, thus assuring almost perfect balance of opposing forces. 

The higher percentage of time spent at top dead centre (dwell) improves theoretical engine efficiency of constant volume combustion cycles and allows the near-elimination of piston skirts and cylinder scuffing, as side loading of the piston is greatly reduced. However, possible increased heat loss during combustion due to extended dwell at top dead centre may have been negated by using a stratified direct injection cycle to reduce heat losses.



Range extender success



ASF Technologies said that the Sytech Scotch Yoke, opposed-cylinder, boxer-type engine is patented and has been tested by VW, Ford, Cosworth, FEV and others. It is the first successful four-stroke Scotch Yoke engine to be specifically designed and developed for modern hybrid and range-extended battery-electric vehicles.



The design has advantages in weight, length, width, height, vibration and emissions over conventional engines in its output class. ASF claims very low NVH that is15 db(A) less than an equivalent boxer engine.

Engine designs include two-, four- and eight-cylinder models, with outputs between 30kW and 220kW.



The engine family nomenclature is based on the number of cylinders and the capacity in litres: a S415 is a four cylinder, 1.5-litre engine that is available in naturally aspirated (NA) and turbocharged (TC) variants.

The Sytech S415TC engine is said to meet Euro 6d and China 6b emissions standards, running on China RON 92 fuel without variable valve timing (VVT) or exhaust gas recirculation (EGR).

Compatible fuels are petrol, liquid petroleum gas (LPG), compressed natural gas (CNG) and hydrogen (H2).