Designing a New 6-Stroke Cycle in a 4-Stroke ICE Using a Water-Vapor Bottoming Cycle

Implemented at the Shahrood University of Technology — Research Center. We re-timed a conventional 4-stroke spark-ignition engine and added two new water-injection/expansion strokes. The idea: after the standard power (combustion) stroke, re-compress the hot exhaust gases and inject high-pressure water. The water rapidly flashes/evaporates, expands, and delivers an additional power stroke. We designed and fabricated a custom timing (cam) shaft, a high-pressure water injection system, and a mobile test rig to evaluate the cycle.

Novel thermodynamic cycle High-pressure atomization Custom camshaft & valve timing

Concept

Keep the standard 4 strokes (intake, compression, combustion/power, exhaust).
Add two strokes:
1. Re-compression of retained hot gases.
2. Water injection & expansion (flash to steam → extra work output).
Timing strategy ensures adequate temperature/pressure for rapid vaporization.

Role & scope

Organization: Shahrood University of Technology — Research Center

My role: Concept, detailed design, fabrication, and test integration.

Hardware: custom multi-lobe camshaft, followers, bearings; high-pressure pump; injectors; control & plumbing; rolling test chassis.

Extended technical report (non-proprietary)

1) Thermodynamic cycle

The 6-stroke sequence modifies the gas exchange and adds a water-vapor bottoming loop: I Intake → II Compression → III Combustion/Power → IV Partial Exhaust (retain hot gases) → V Re-compression of trapped residuals → VI Water injection & steam expansion (second power stroke). The residual gas temperature after IV–V is leveraged to flash atomized water to vapor. Valve phasing is tuned to trap sufficient enthalpy while limiting pumping loss.

2) Valve timing & camshaft

We replaced the OEM cam with a custom multi-profile shaft to realize atypical hold-open/early-close events for the re-compression and water strokes. The design process included: (i) lobe-profile synthesis (dwell, ramp, jerk limits for follower survivability), (ii) bearing selection and seat drawings, (iii) CNC machining, (iv) assembly and run-out checks. Photos (below) show CAD, mechanical drawings, raw lobes, and the final shaft.

3) Water injection system

To achieve rapid vaporization, we employed high-pressure liquid injection with fine atomization. The system includes a plunger-type pump, accumulator, stainless lines, and a single-hole/mini-multihole injector. Injection is phased late in re-compression with the intake closed to maximize sensible heat transfer and minimize wall wetting. Nozzle targeting and droplet SMD were selected to avoid quench and to preserve wall films.

4) Controls & instrumentation

A simple phase controller synchronized cam angle, ignition, and water-pulse timing. On-engine sensors measured manifold pressure, crank speed, injector pressure, and exhaust temperature. The rolling rig allowed safe outdoor testing with radiator and cooling loop mounted on the frame.

5) Test observations

Clean steam puffs observed during water-strokes at correct phasing and pressure.
Measured torque rise during water-expansion compared to dry baseline at same throttle.
Knock not observed due to water’s charge-cooling; peak exhaust temperature reduced.
Key sensitivities: injector pressure & droplet size, re-compression pressure, and trapped residual mass.

6) Practical considerations

Materials: corrosion-resistant lines/nozzles; attention to water/oil separation.
Thermal management: avoid cylinder wall quench; favor mid-chamber spray targeting.
Safety: high-pressure water line shielding; over-pressure relief; hot-surface spray protocols.

7) Status & outcomes

Demonstrated feasibility on a small SI engine with custom timing hardware.
Built a reusable research platform for further optimization (EGR fraction, multi-pulse, variable water rate).
Documentation package: CAD, drawings, timing maps, and test logs.