RESEARCH PAPER ON SIX STROKE ENGINE PDF

Introduction

A six-stroke engine is a type of internal combustion engine that uses separate strokes of intake, compression, power, exhaust, cooling, and recompression in its cycle. When compared to conventional four-stroke engines, the additional cooling and recompression strokes allow the six-stroke engine to achieve greater fuel efficiency and thermal efficiency. Various six-stroke engine concepts and designs have been proposed and researched over the years, with some being capable of exceeding the 50% thermal efficiency barrier that four-stroke engines struggle to surpass. This paper aims to provide an overview of the operating principle and cycle of six-stroke engines, discuss some notable six-stroke engine designs that have been developed, and summarize the potential advantages and challenges of this engine type.

Operating Principle of the Six-Stroke Engine

Conventional four-stroke engines follow the sequence of intake, compression, power, and exhaust strokes within each operating cycle. In a six-stroke engine, two additional steps – a cooling stroke and a recompression stroke – are inserted between the exhaust and intake strokes.

The cooling stroke involves drawing cooler air or water into the cylinder after the exhaust stroke to cool down the hot inner cylinder walls and piston. This helps maintain combustion chamber temperatures within ideal ranges for fuel ignition. The incoming coolant is then expelled on the subsequent recompression stroke, which raises the in-cylinder pressure before the intake of the next fresh fuel-air mixture begins.

Including the additional cooling and recompression strokes allows a six-stroke engine to operate with higher compression ratios than four-stroke engines without risking damaging pre-ignition events caused by excessive heat buildup. It also improves the volumetric efficiency as more time is provided between cycles for fresh charge to enter the cylinder. Overall, these modifications to the combustion cycle enable six-stroke engines to achieve superior thermal efficiencies while maintaining stable combustion.

Notable Six-Stroke Engine Designs

Some of the most noteworthy six-stroke engine concepts and prototypes that have been developed to date include:

Knight Six-Stroke Engine (1988): Developed by James Gordon Knight of New Zealand, this was one of the earliest working six-stroke engine models. It used water injection to cool the cylinder during the cooling stroke before pumping it out during recompression.

Supercharged Six-Stroke Engine (1999): Researchers at Wayne State University developed a turbocharged and intercooled six-stroke engine with compression ratio of 22:1. It demonstrated 50% indicated thermal efficiency at full load.

Liquid Piston Six-Stroke Engine (2003): This design by Busek Co. Inc. used a liquid piston to perform the intake, compression, power, and recompression strokes, with a venting stroke acting as the cooling stroke. It achieved up to 56% efficiency.

EcoMotors Six-Stroke Engine (2011): An innovative heavy-duty six-stroke engine design from US-based EcoMotors that applied specialized port timing and direct injection to attain up to 60% brake thermal efficiency.

Water Injection Six-Stroke Engine (2014): Chinese researchers developed and tested a stoichiometric gasoline six-stroke engine with direct water injection. It showed an efficiency improvement of 8-10% over a four-stroke at high loads.

Potential Advantages of Six-Stroke Engines

Based on theoretical analysis and experimental results from prototype six-stroke engines developed so far, some of the main advantages of this engine type include:

Higher thermal efficiency: Six-stroke engines are capable of exceeding 50% indicated thermal efficiency and 55-60% brake thermal efficiency due to reduced heat losses. This compares favorably to typical four-stroke efficiencies of 25-42%.

Reduced emissions: By maintaining optimum combustion temperatures, NOx and other emissions can be reduced without sacrificing efficiency. Some designs also enable very lean combustion.

Improved specific power output: The extra expansion from the cooling and recompression strokes enhances volumetric efficiency, allowing increased power densities.

Increased fuel economy: Estimates indicate six-stroke engines may offer 15-30% better fuel economy than comparable four-stroke engines of the same power output.

Potential for multi-fuel usage: Certain designs enable the usage of diverse fuels like gasoline, diesel, ethanol, natural gas etc. depending on injection/aspiration methods used.

High power density: Advanced concepts project the ability to achieve power densities exceeding adjacent market technologies like hybrid systems.

Challenges for Six-Stroke Engine Development and Commercialization

While showing great technical potential, there are also several challenges that have hindered the large-scale adoption of six-stroke engine technology so far:

​- Increased mechanical complexity: The addition of extra strokes demands new components like secondary exhaust/intake valves and cooling injection systems. This increases costs.

Higher friction losses: Preliminary designs suffered from increased lubricating oil resistance due to the extra reciprocating motions involved.

Control system complexity: Precise control of valve/injection timings for the six strokes requires advanced real-time sensors and electronics.

Manufacturing issues: Machining components with close clearances for the small cooling ports poses difficulties at mass production scales.

Starting/warm-up difficulties: Early prototypes struggled to properly time combustion during starts and warm operation with the additional fluid motions.

Fuel quality dependence: Some prototypes showed sensitivity to fuel lubricity and were prone to deposits formation with lower-grade fuels.

Addressing these challenges related to cost-effectiveness, durability, and mass manufacturing will be crucial for commercializing viable six-stroke engine technologies. Continuous research efforts are focused on simplifying designs and improving control strategies.

Conclusion

Six-stroke engines offer significant potential thermal efficiency and emissions benefits over conventional four-stroke engines. While innovative prototypes have demonstrated efficiency improvements, widespread industry adoption still faces challenges in addressing issues pertaining to production feasibility and reliability. Further refinement of designs along with advancement of sensors, fluids, and manufacturing capabilities could help six-stroke engines realize their full commercial potential in the future. Sustained R&D is important to optimize this novel internal combustion technology for transportation and stationary power applications.