The Design, Development, and Preliminary Results from a High-speed, Optically Accessed, Single-cylinder Engine
With increasing legislation on emissions, greater expectations in performance and fuel economy and an ever decreasing time required from concept to production engines, the necessity to understand the fundamental principles in all aspects of engine performance has never been higher. Lotus has designed and built an optically accessed single cylinder research engine to allow observation and measurement of the different phenomena occurring inside the engine cylinder. The engine has a full transparent fused silica cylinder, so the whole engine stroke can be observed, and a sapphire window in the piston crown, which can be accessed via a 45 degree mirror in an extended piston.
Manufacturers are continually being driven to improve the combustion process of the internal combustion engine to reduce engine-out emissions and fuel consumption by ever more stringent emissions legislation, environmental, and customer pressures.
A number of new technologies have become available that appear to offer manufacturers the benefits and improvements they seek. Amongst these technologies are:
- Gasoline direct injection. GDI technology has been demonstrated to offer lean burn fuel economy benefits and the possibility of reduced emissions during transient and cold start operating conditions due to better control of fuel–air mixing (1, 2).
- Controlled auto ignition. AI technology has been shown to give benefits to fuel consumption, emission formation, and engine stability (3).
Many of these new technologies require detailed understanding of the combustion process and unique architectures of the combustion system to ensure correct and robust combustion. These solutions can no longer be derived from the years of existing development data and experience gained from conventional manifold or port fuelled homogeneous combustion systems.
A further pressure on the manufacturer is to remain competitive with reduced time-to-market and aggressive timing associated with today's rapidly changing market place. In order to meet these demands manufacturers must have access to a means to rapidly develop new combustion strategy, and thus, enable effective design of improved combustion systems.
To further advance its powertrain research and development activities Lotus Engineering has designed a single-cylinder, high-speed, optical engine with full optical access and identical internal geometry. With un-obscured optical access up through the piston crown, cylinder bore, and pent-roof, this engine allows full use of a wide range of optical techniques including laser induced fluorescence (LIF), phase Doppler anemometry (PDA), and high-speed imaging. The authors believe that this latest generation of optical engines will move from the pure research field into the early phase of engine development programmes. This will give engineers the ability to combine laser diagnostic techniques and calibration skills to ensure new robust combustion systems are developed and can be applied to real specific engine models in acceptable time frames for the engines of the future.
Optical diagnostic techniques can allow data to be gained on in-cylinder phenomena involving interactions of fuel and air and the formation of combustion products. Data on combustion processes, ignition, flame kernel growth and propagation, flame structure, soot formation, and wall quenching can also be gained.
There are several, sometimes conflicting, requirements that the engine has been designed to meet. The most basic of these is that the single-cylinder engine should mimic the geometry and performance, in terms of speed and load, of a modern multi-cylinder petrol engine. Added to this capability is also the necessity to be able to represent small high-speed direct injection diesel engines with their increased compression pressures. These criteria have been met by designing the engine with primary and secondary balancing to run at speeds greater than 5000 r/min under full load conditions for a petrol engine, with the glass components being designed to handle the pressures and forces generated. Design flexibility has been built into the engine, to require only minimum modification, allowing any engine geometry up to a bore and stroke of 100 mm by 100 mm to be used, and fast prototyping of the head geometry to keep as close as possible to the actual engine configuration.
These most basic design criteria have to be complementary to the optical access demanded by the various laser diagnostic and imaging techniques. The requirement was to have full access to the engine cylinder, Fig. 4.1, including any pent roof, and this was achieved by having the complete cylinder built of fused silica. Additionally, access was required through the piston crown, and here an optical sapphire window was fitted into the top of the piston. Even with the optical windows in place, additional care had to be taken in the design of the peripheral components, i.e. timing belts, to ensure that these did not interfere with the access so generated. This access is not only a question of being able to see all parts of the cylinder, but imposes further constraints when high-powered lasers are utilized. There is then the necessity of running the engine optical cylinder dry, i.e. oil free, to prevent oil being burnt on to the glass surface and also giving an unknown refractive effect to the laser beams transmission due to the presence of oil films.
The final requirement was one of maintenance, for both the mechanics of the engine and also for mounting/de-mounting the optical components. The challenge here was designing the engine in such a way that the optical components could be removed very quickly for cleaning, and then rebuilt into the engine, to minimize the down time during experimental work. The object was to ensure that this down time did not become the dominat time during the investigative work.
The main optical component is the fused silica cylinder in which the special piston runs. This allows optical access to the full bore and stroke. Principle parameters considered during its design were its strength to withstand high cylinder pressures and the optical suitability of its geometry and material. A hydraulic platform is used to hold the cylinder up to the head to facilitate removal and replacement of the optical liner. Increasing cylinder wall thickness distorts light passing through it. Thinner walls however limit the maximum pressure that can be used. Thermally toughened glasses cause distortion of polarized coherent laser beams used in phase Doppler anemometry and chemically toughened glasses can absorb and fluoresce under high-power UV wavelength lasers used in Laser Induced Fluorescence. Although not the strongest optical material available, fused silica was chosen for a general-purpose cylinder due to its good optical and mechanical properties and relative cost. The cylinder has a wall thickness of 14 mm, calculated to allow cylinder pressures of up to 40 bar to be used with a significant safety factor. The top of the optical cylinder and its mating surface in the cylinder head has a specially developed contoured profile to provide optical access directly into the pent-roof combustion chamber from the side without compromising the valve seat or port geometry.
A circular window made of 12 mm thick hem-sapphire crystal (Al2O3) is mounted in the crown of the piston. Every effort was made to maximize the useful optical area while minimizing weight and stress concentration. The final design ‘encases’ the sapphire window with titanium whose coefficient of linear thermal expansion is almost identical to that of the sapphire itself. The conical section window is fixed to the piston body by a titanium retainer with a mating cone shape. The window is separated from the piston body by a titanium washer. This window allows access up to the cylinder head and the valve scat area, Fig. 4.2.
The cylinder head was machined from cast iron, which was selected as a material for its rigidity and thermal stability over aluminium. The port and valve seat geometry has been detailed to ensure the same air motion characteristics as the engine that formed the basis for this design. The cylinder head design is sufficiently modular and flexible that alternative port geometry ca be manufactured and evaluated rapidly with minimum engine down time in order to ensure optimum air and fuel motion is achieved.
The cylinder head is mounted on four columns above the optical section of the engine. These columns are spaced in such a way that there is no obstruction to the various ‘optical envelopes’ required for planar imaging and scanning single point measurements from the whole bore and stroke. Two camshafts in separate housings actuate the valve gear. The camshaft housings are readily removable and are to be replaced by the Lotus Active Valve-Train system to give the engine a fully variable hydraulically actuated valve system.
The cylinder block consists of two sections. The upper section provides mounting for the cylinder head, hydraulic platform, 45 degree mirror, and steel cylinder. This mounts on to a machined-from-solid aluminium lower crankcase containing a cross-drilled crankshaft running on three pressure fed main bearings.
Although the elongated piston is manufactured from lightweight materials, the reciprocating mass at 1235 gms is considerable when compared to that of an equivalent production piston at 300–400 gms. As the engine was intended for high-speed use, and to be used in close proximity to mechanically sensitive optical devices, it was decided that the added cost and complexity of adding balance shafts was justified. Two contra-rotating primary and secondary balance shafts with adjustable balance weights are built into the crankcase and are gear driven from the crankshaft.
An elongated and bifurcated piston design was adopted, as has become common practice in optical engines to allow optical access into the combustion chamber through the piston via a fixed 45 degree mirror.
The design allows the glass liner and piston crown to be removed quickly without dismantling the engine. The glass liner is first lowered on its hydraulic platform and the piston gudgeon pin is removed through an aperture in the crankcase. This allows the piston to be lowered to clear the optical cylinder allowing the glass to be removed from its seat. With the cylinder removed the piston crown can be unscrewed from the piston assembly and the window cleaned or replaced. To minimize the reciprocating mass resulting from the long piston assembly and window, advanced materials were considered. However, aluminium and titanium were selected for ease of manufacture and thermal compatibility with sapphire.
The piston was designed to run un-lubricated in the glass bore and at the high design speeds sealing without excessive friction was a concern. A considerable amount of rig testing was carried out to identify the best material for the piston compression ring. A carbon/carbon matrix was found to be the best material exhibiting both low friction and high temperature capability.
The base measurement set required for future work with this research engine, is the in-cylinder air flow generated while the engine is being motored. This data is used for comparison with regard to fuel flow and combustion propagation. The airflow was measured using laser Doppler anemometry, and the first data set was collected with the engine running at 1500 r/min. Three vertical planes were measured; one diameter between the inlet valves, the orthogonal Diameter, and one further plane through the centre of one of the inlet valves, to measure the maximum tumble component. The horizontal scans were performed with a 5 mm matrix of points, with 10 mm between each vertical location. At each position the axial and radial components of velocity were measured. It is planned to measure a few horizontal planes, where the swirl/radial components will be measured, giving some locations where all three velocity components will be available. The velocity vectors, measured on a plane through the centre of the cylinder and in between the inlet valve pair, i.e. the symmetry plane, are shown in Fig. 4.3.
These vector fields indicate how the inlet flow develops and then decays by bottom dead centre. There is also evidence of the three-dimensional nature of the flow field, most noticeably where adjacent vectors are directly opposed. The flow fields and subsequent tumble analysis are fully described in (7).
Although the engine has been designed with direct injection as one of the main operating conditions for research, there was also a requirement to run the engine with port injection. For this purpose the standard production injector was mounted into the inlet manifold, directed on to the rear of the inlet valves and injected gasoline with 3 bar pressure. The principle object of this work was to capture some combustion images with the engine running in conventional spark ignition and auto-ignition mode. It also served as an opportunity to prove the engine design in firing mode. The first images were obtained with the engine operating with convention spark ignition, at 1000 r/min, and an example image is shown in Fig. 4.4. The camshafts were then substituted to allowed internal exhaust gas trapping and the engine was then run at 2000 r/min. Under these conditions the engine can be made to run with auto-ignition. In this mode of operation, there are multiple ignition sites, and these are shown in Fig. 4.5. It is these multiple sites that are believed to be responsible for the reduction in peak pressure fluctuations observed in this mode of operation.
This Chapter has described the design, major components, and early commissioning of a single-cylinder research engine for laser diagnostics. The engine has been built to allow laser diagnostics to be used to investigate in-cylinder flow structure, air fuel mixing, and combustion, has finished its commissioning and is used on a day-to-day basis. Initial work has been to characterize the in-cylinder air motion, using laser Doppler anemometry, as a basis for further research into airflow/spray interaction and combustion. The engine has also been fired in both spark-ignition and auto-ignition modes, and combustion images obtained. Future work will be looking at the break-up of direct injection sprays in the engine cylinder for both homogeneous and lean burn operation.
(1) Wigley, G., Hargrave, G., Law, D., Pitcher, G., Durell, E., and Allen, J. Air Flow and Fuel Spray Characterisation – Diagnostics for 21st Century Engines, Proceedings of the 21st Century Emissions Conference, IMechE, London, UK, December 2000.
(2) Law, D., Kemp, D., Allen, J., Kirkpatrick, G., and Copland T. Controlled Combustion in an IC Engine with a fully Variable Valve Train, Proceedings of the SAE 2001, Advances in Combustion, Detroit, USA, March 2001.
(3) Law, D., Allen, J., Kemp, D., and Williams, P. 4 Stroke Active Combustion (Controlled Auto-Ignition) Investigations using a Single Cylinder Engine with Lotus Active Valve Train (AVT), Proceedings of the 21st Century Emissions Conference, IMechE, London, UK, December 2000.
(4) Pitcher, G. and Wigley, G. LDA Analysis of the Tumble Flow Generated in a Motored 4 Valve Engine, Ninth International Conference Laser Anemometry Advances and Applications, University of Limerick, Ireland, 2001.
Maly, R. R. Progress in Combustion Research. IMechE Combustion Engines Group Prestige Lecture 8/10/98.
Kuwahara, K. et al. Mixing Control Strategy for Engine Performance Improvement in a Gasoline Direct Injection Engine, Proceedings of the SAE 1998, Advances in Combustion, Detroit, USA, 1998, SAE report number 980158.
Kuwahara, K. et al. Diagnostics of In-Cylinder Flow, Mixing and Combustion in Gasoline Engines – Proceedings of VSJ-SPIE98 6-9/12/98.
G Pitcher, P Williams, and J Allen
Lotus Engineering, Norwich, UK
Loughborough University, Loughborough, UK
© With Authors 2002