Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

82 June/July 2017 | Unmanned Systems Technology PS | Mass-market HCCI D eveloping propulsion systems for UAVs is a challenging task given that their operating requirements – high power-to-weight ratio, long range, high load capacity and so on – require high efficiency from the propulsion system (writes Stewart Mitchell). If the chosen method is an internal combustion (IC) piston engine then the combustion efficiency has to be maximised, and that is a function of the thermal characteristics of the combustion in the cylinder. To that end, the most efficient combination is a high compression ratio with a lean mixture, with the mixture being burnt as rapidly as possible. At least that’s the theory. This is where homogenous charge compression ignition (HCCI) comes in. In short, with HCCI a uniformly dispersed air-fuel mixture is compressed inside a cylinder to the point of instantaneous auto-ignition, to enable lean-burn operation and high thermal efficiency. It is applicable to all systems that need highly efficient IC engine powertrains, including unmanned vehicles. Most current HCCI engines use very complicated componentry, such as cylinder heads with built-in pre- combustion chambers and electronically actuated control systems that accurately monitor and control the temperature of the working fluids throughout the intake and compression cycle. However, last year US-based engine development company Nautilus Engineering introduced an HCCI base engine design model which it says overcomes the issues associated with bespoke and complicated HCCI systems. That means HCCI can feasibly be introduced to the mass market. The Nautilus engine concept has a conventional piston-in-cylinder design with poppet valves that open and close passages into the cylinder, but that is where the similarities between it and standard IC engines ends. Its twin-cylinder boxer engine features a piston measuring 3.25 in in diameter with a mini-piston with a 1 in diameter projecting 0.75 in up on the central axis. When the piston approaches top dead centre, the smaller piston travels into a mini-cylinder area in the cylinder head with a 0.5 mm clearance around the circumference. This small area creates what Nautilus calls the primary combustion chamber; the rest of the piston area creates a secondary combustion chamber. And because the engine uses compression ignition, it operates without a spark plug. “The primary chamber geometry has a compression ratio three times that of the main [secondary] combustion chamber [24:1 compared to 8:1], “ explains the company’s CEO Matthew Riley. “As the air-fuel mixture is compressed, it ignites by compression first in the primary chamber and propagates the air-fuel detonation via blow-by around the perimeter of the primary piston to the larger secondary chamber. “The combustion in the primary chamber reaches 1300 psi and, as it expands into the secondary chamber area – which is around 160 psi – the pressure rise is so high and fast that it generates full instantaneous expansion of the homogenous mix in the secondary space.” The engine operates at an air-fuel ratio ranging from 24:1 to 31:1, depending on engine load. Riley says combustion temperatures are expected to be around 650 C – far below the NO X generation temperature of conventional gasoline, therefore reducing emissions as well. At the time of writing, the engine existed only in CAD form, but with some promising figures coming from simulations, that looks set to change. Now, here’s a thing “ ” The engine concept has a conventional piston-in- cylinder design, but that is where the similarities end