/projects
REV 2-Stroke Engine
Updates
Overview
This is currently my most active project. It investigates a non-traditional approach to improving the emissions and efficiency of the 2-stroke engine.
The 2-stroke seemed to reach its peak in its final years in MotoGP before the now-more-common 4-stroke took over. Recently, however, several new technologies have created a niche comeback for the engine type. Transfer-port injection, electronic fuel and oil injection, and electronically controlled exhaust valves have all brought new attention to the 2-stroke, including my own.
This project focuses on combining transfer-port fuel injection with a crank-driven rotary exhaust valve, or REV, to create asymmetric exhaust timing.
The Problem with the Conventional 2-Stroke Cycle
In a traditional crankcase-scavenged 2-stroke engine, the transfer ports deliver a fresh air-fuel mixture while the exhaust port is still open. Some of this fresh charge can travel directly across the cylinder and escape into the exhaust before it becomes trapped.
The tuned expansion chamber can return some of this mixture through a reverse pressure wave, which also contributes to the recognizable sound and power characteristics of a 2-stroke. However, this process remains highly dependent on engine speed and exhaust tuning, thus the reason for a short, tight powerband.
The Short-circuiting of the fresh charge contributes to high unburned hydrocarbon emissions and increased fuel consumption. These are some of the most common weaknesses of the conventional 2-stroke engine.
Proposed Engine Concept
To solve some of these issues, the proposed engine does not use a fundamentally different thermodynamic cycle. Instead, it changes the gas exchange and fuel delivery processes of a crankcase scavenged 2-stroke engine.
The rotary exhaust valve is positioned directly downstream of the cylinder exhaust port. It is placed as close to the cylinder as possible to minimize the crevice volume between the piston-controlled exhaust port and the valve. Fuel Injectors are also placed in the transfer ports to fire at a controlled point. They are placed here to be able to control the amount of fuel entering the cylinder much more closely and reduce wall wetting through the crankcase.
The addition of the REV allows the effective exhaust closing event to occur independently of the piston-controlled exhaust port. The physical exhaust port can remain open while the rotary valve closes the passage between the cylinder and the exhaust system. This creates asymmetric exhaust timing. The exhaust system can open early for blowdown but close much earlier than would be possible using piston control alone. The actual engine cycle is explained below:
1. Expansion: TDC to EPO
Combustion begins near top dead center and produces the pressure that drives the piston downward during the expansion stroke.Throughout most of this period, the cylinder exhaust port remains covered by the piston. As the piston approaches exhaust port opening, the REV rotates into its fully open position.
The valve should provide its maximum effective flow area when the piston first uncovers the exhaust port so that it does not restrict the beginning of the blowdown process.
2. Blowdown: EPO to TPO
At exhaust port opening, the piston uncovers the cylinder exhaust port while the REV is fully open. The high pressure combustion gases begin discharging from the cylinder into the exhaust system.The purpose of blowdown is to reduce the cylinder pressure below the pressure available in the crankcase and transfer passages before the transfer ports open.
During this interval, the transfer ports remain closed, so the exhaust system is the only major flow path out of the cylinder.
3. Air Scavenging: TPO to REVC
At transfer port opening, the piston uncovers the transfer ports. The cylinder exhaust port, transfer ports, and REV are now open at the same time.Fresh air enters the cylinder through the transfer passages and pushes the remaining combustion gases toward the exhaust port. This is the primary loop-scavenging period and initially operates in the same way as a conventional loop-scavenged 2-stroke engine.
The main difference is that the transfer passages supply primarily air rather than a premixed air-fuel charge. This allows the exhaust system to remain open during the most useful part of scavenging without allowing a large amount of fuel to escape directly into the exhaust.
The REV remains fully or substantially open through blowdown and the early scavenging period. Its closing point must be selected carefully so that it does not interrupt usfuel scavenging.
4. Exhaust Closure and Fuel Injection: REVC to TPC
At or shortly after bottom dead center, the rotary exhaust valve closes. The piston-controlled exhaust port may still be physically uncovered, but the closed REV blocks the passage between the cylinder and the exhaust system. This creates a fueling period during which the transfer ports remain open while the exhaust passage is closed. Fuel is injected into the transfer passages and carried into the cylinder by the remaining transfer flow. The goal is not necessarily to retain every individual fuel droplet. Instead, the objective is to greatly reduce fuel short-circuiting.
This is one of the most important and difficult stages of the proposed cycle. After bottom dead center, the piston begins moving upward, and the cylinder volume decreases while the exhaust system is closed. As a result, the cylinder pressure begins to rise while the transfer ports are still open. If the cylinder pressure rises above the crankcase pressure, flow can reverse from the cylinder back into the transfer passages. The interval between REV closing and transfer port closing must therefore be optimized to provide:
- Enough time to inject the required fuel mass
- Continued positive transfer flow, or an acceptably small amount of reverse flow
- Adequate fuel atomization and mixture distribution
- Minimal fuel loss into the exhaust
- An acceptable trapped residual-gas concentration
- A suitable final air-fuel ratio
5. Compression and Mixture Preparation: TPC to TDC
At transfer port closing, or TPC, the rising piston covers the transfer ports. The cylinder is now isolated from both the transfer system and the exhaust system. The trapped air, fuel, and residual exhaust gases are compressed as the piston continues toward TDC. During this period, the fuel must continue to atomize and mix with the trapped air.
The remaining in-cylinder motion from the scavenging process should help distribute the fuel and prepare the mixture for ignition. Combustion then begins near top dead center, and the cycle repeats.
The Current State
This project is ongoing, so its state is always changing. That said, this project is still in the computational 1D/0D phase with hopes of moving to rough CAD and OpenFOAM CFD soon. A complete project map can be found here or is listed in the updates section above. As progress continues, I will make sure to occasionally post any content from the project.
Long-Term Goal
The long-term goal is to manufacture and test a working prototype of this engine. With complex parts becoming easier and more affordable to produce, I believe there is now a realistic path to prototyping this design faster and at a lower cost than would have been possible in the past.
I cannot say with any confidence that the first version of this engine will be successful enough to have a practical use, but that is not the goal of the project. The goal is to design, build, and test a complete prototype, then use what I learn from that process to identify problems, improve the design, and continue iterating.
It is easy to become caught up in the idea of building an engine that becomes a memorable machine, but that is not what I expect from this project. I would already consider it a major accomplishment to reach the first working prototype. Achieving even a portion of the original performance and efficiency goals would be an incredible result.
At its core, this is a passion project meant to challenge me.