A new design concept for 2-Stroke aircraft Diesel engines

High power density, low weight, compact dimensions, high efficiency as well as reliability are the key factors in designing and dimensioning piston engines for General Aviation and Unmanned Aerial Vehicle (UAV) power plants. Despite of new available technologies, conventional solutions are still struggling to fulfill simultaneously all those requirements. The paper explores the application of a new design of 2-Stroke externally scavenged engines to aircraft. The new concept basically consists in the use of a patented rotary valve for controlling the flow through a set of inlet ports, enabling supercharging and the achievement of extremely high power densities compared to conventional solutions. The scavenging is realized by using an external pump, made up of a further cylinder, whose piston is connected to the same crankshaft. The piston pump allows the crankcase to be used as a conventional oil sump, and greatly improves the crankshaft balance. No poppet valves or camshafts need to be installed, since the flow is driven by piston-controlled ports and by two sets of reed valves. The engine can adopt two types of combustion system: Gasoline Direct Injection (GDI) for SI operations, and Direct Injection Common Rail for Diesel cycle. The paper is focused on the last version, since it can run on standard aircraft fuel. The Diesel engine has three cylinders and three piston pumps, for a total displacement of 1.5 liter The engine is turbocharged and inter-cooled, in order to reach a power target, at sea level, of 150 kW@4000 rpm. Another fundamental target is the minimum power of 100 kW, at the altitude of 20,000 feet. The paper reviews the design of the engine and presents the numerical prediction of the key performance parameters

Combustion analysis of a diesel engine running on different biodiesel blends

Rape-seed biodiesel is an interesting option to address the problem of decreasing availability of conventional fossil fuels, as well as to reduce the CO2 emissions of internal combustion engines. The present paper describes an experimental campaign carried out on a current production 4-cylinder, 4-stroke naturally aspirated diesel engine, running on standard diesel fuel and on three different blends of rape-seed biodiesel (20%-50%-100%). Performance, emissions and in-cylinder pressure traces were measured at full load. It was found that the influence of rape-seed biodiesel in the fuel blend is not constant at each operating condition. However, as the biodiesel content increases, full load performance tends to drop, in particular brake specific fuel consumption (maximum worsening: +18%), while soot emission goes down. The maximum improvement observed in terms of soot concentration is 37.5%, at 1200 rpm. The combustion analysis revealed that the main differences among the fuels occur in the first phase of combustion: the burn rate is slower for biodiesel blends at low speeds, and faster at high

CFD-3D Analysis of a Light Duty Dual Fuel (Diesel/Natural Gas) Combustion Engine

AbstractNowadays, the most critical issues concerning internal combustion engines are the reduction of the pollutant emissions, in particular of CO2, and the replacement of fossil fuels with renewable sources. An interesting proposition for Diesel engines is the Dual Fuel (DF) combustion, consisting in the ignition of a premixed charge of gaseous fuel (typically natural gas) by means of a pilot injection of Diesel Fuel.Dual fuel combustion is a quite complex process to model, since it includes the injection of liquid fuel, superimposed with a premixed combustion. However, CFD simulation is fundamental to address a number of practical issues, such as the setting of the liquid injection parameters and of the gaseous fuel metering, as well as to get the maximum benefit from the DF technique.In this paper, a customized version of the KIVA-3V Computational Fluid Dynamic (CFD) code was adopted to analyze the combustion process of a 4-cylinder, 2.8 l, turbocharged HSDI Diesel engine, operating in both Diesel and DF (Diesel and Natural Gas) modes.Starting from a previously validated diesel combustion model, a natural gas combustion model was implemented and added to simulate the DF operations. Available engine test data were used for validation of the diesel-only operation regimes. Using the calibrated model, the influence of the premixed charge composition was investigated, along with the effect of the diesel injection advance angle, at a few characteristic operating conditions. An optimum setting was eventually found, allowing the DF engine to deliver the same brake power of the original Diesel unit, yielding the same maximum in-cylinder pressure.It was found that DF combustion is soot-less, yields a strong reduction of CO and CO2, but also an increase of NO

Combustion System Development of an Opposed Piston 2-Stroke Diesel Engine

Today, the interest towards 2-stroke, opposed-piston compression-ignition engines is higher than ever, after the announcement of imminent production of a 2.7L 3-cylinder light truck engine by Achates Powers. In comparison to other 2-stroke designs, the advantages in terms of scavenge and thermal efficiency are indisputable: a perfect "uniflow" scavenge mode can be achieved with inexpensive and efficient piston controlled ports, while heat losses are strongly reduced by the relatively small transfer area. Unfortunately, the design of the combustion system is completely different from a 4-stroke DI Diesel engine, since the injectors must be installed on the cylinder liners: however, this challenge can be converted into a further opportunity to improve fuel efficiency, adopting advanced combustion concepts. This paper is based on a previous study, where the main geometric parameters of an opposed piston engine rated at 270 kW (3200 rpm) were defined with the support of CFD 1D-3D simulations. The current work will focus on the influence of an innovative combustion system, developed by the authors by means of further CFD-3D analyses, holding constant the boundary conditions of the scavenging process. The numerical study eventually demonstrates that an optimized 2-S OP Diesel engine can achieve a 10% improvement on brake efficiency at full load, in comparison to an equivalent conventional 4-stroke engine, while reducing in-cylinder peak pressures and turbine inlet temperatures

Two-Stage Turbocharging for the Downsizing of SI V-Engines

AbstractOne of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking.This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range.It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intake-exhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered.The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool.The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine

Design Of Two-Stage On/Off Cartridge Valves For Mobile Applications

Cartridge valves are widely used in mobile applications, where they are screwed in manifolds, to realize opportune circuit layouts. These valves are quite simple in operation but require a sophisticated design in order to meet all the requirements needed in the mobile machines. Typically, the design process is developed realizing a first design concept and some prototypes and experimentally testing them; after this, the designer chases the optimal performances requested to the valve with a trial and error approach on the prototypes, involving high time and cost resources. In this paper an alternative design procedure is proposed, which involves dedicated simulations to analyze the main critical issues regarding the cartridge valve object of the study. Modelling and simulations here have been considered as steps into the design process of a new valve, which satisfies the requirements and well adapt to the necessities to operate at higher flow and pressure levels without compromising its performances. In that way, the number of prototypes, realized to validate the numerical results and verify the design process, has been considerably reduced, together with related time and costs

Pressure losses in hydraulic manifolds

Hydraulic manifolds are used to realize compact circuit layout, but may introduce a high pressure drop in the system. Their design is in fact oriented more toward achieving minimum size and weight than to reducing pressure losses. This work studies the pressure losses in hydraulic manifolds using different methods: Computational Fluid Dynamic (CFD) analysis; semi-empirical formulation derived from the scientific literature, when available; and experimental characterization. The purpose is to obtain the pressure losses when the channels' connections within the manifold are not ascribable to the few classic cases studied in the literature, in particular for 90° bends (elbows) with expansion/contraction and offset intersection of channels. Moreover, since CFD analysis is used to predict pressure losses, general considerations of the manifold design may be outlined and this will help the design process in the optimization of flow passages. The main results obtained show how CFD analysis overestimates the experimental results; nevertheless, the numerical analysis represents the correct trends of the pressure losses

Design and experimental development of a compact and efficient range extender engine

The paper reviews the design and experimental development of an original range-extender single-cylinder two-stroke gasoline engine, rated at 30 kW (maximum engine speed: 4500 rpm). The goal of the project is to get most of the benefits of the two-stroke cycle (compactness, high power density, low cost), while addressing the typical issues affecting the conventional engines of this type. Among many recent similar propositions, the peculiarities of this engine, besides the cycle, are: external scavenging by means of an electric supercharger, piston controlled scavenge and exhaust ports (no poppet valves), gasoline direct injection (GDI), and a patented rotary valve for the optimization of the scavenging process, of the loop type. Lubrication is identical to a conventional four-stroke engine, and the rotary valve, connected to the crankshaft, helps to improve the balance of the piston reciprocating forces, yielding an excellent NVH behavior. It should be noted that, except the patented rotary valve, all the engine parts are standard automotive commercial components, that don’t require any specific expensive technology. In fact, the originality of the engine consists in the optimum combination of existing well assessed concepts. The scavenging and combustion systems of the engine are developed in the first phase of the project, including the construction and the experimental testing of a prototype. In the second phase, the air metering system of the prototype is completely modified: the piston pump is replaced by an electric supercharger, and engine load is now controlled by the supercharger speed, without throttle valve. The new engine is compared to a standard 4-stroke engine, developed in a previous project for the same application. The main advantages of the two-stroke engine may be summarized as follows: lower weight (−35%), higher brake efficiency (+6%, on average), less heat rejected (−18%), lower thermal and mechanical loads within the cylinder (−40%). The only concern, that will be addressed in a future phase of the study, is the compliance with very low NOx limits: in the worst scenario, the 2-stroke engine could be forced to adopt a well assessed but expensive after-treatment device