Schlieren-based temperature measurement inside the cylinder of an optical spark ignition and homogeneous charge compression ignition engine

Abstract

Schlieren [1, 2] is a non-intrusive technique that can be used to detect density variations in a medium, and thus, under constant pressure and mixture concentration conditions, measure whole field temperature distributions.The objective of the current work was to design a schlieren system to measure line-of-sight averaged temperature distribution with the final aim to determine the temperature distribution inside the cylinder of Internal Combustion engines. In a preliminary step, we assess theoretically the errors arising from the data reduction used to determine temperature from a schlieren measurement and find that the total error, random and systematic, is less than 3%, for typical conditions encountered in the present experiments. A Z-type, curved-mirror schlieren system was used to measure the temperature distribution from a hot air jet in an open air environment in order to evaluate the method. Using the Abel transform, the radial distribution of the temperature was reconstructed from the line of sight measurements. There was good agreement in the peak temperature between the reconstructed schlieren and thermocouple measurements. Experiments were then conducted in a four-stroke, single-cylinder, optical spark ignition engine with a four-valve, pentroof-type cylinder head to measure the temperature distribution of the reaction zone of an iso-octane-air mixture. The engine optical windows were designed to produce parallel rays and allow accurate application of the technique. The feasibility of the method to measure temperature distributions in IC engines was evaluated with simulations of the deflection angle combined with equilibrium chemistry calculations that estimated the temperature of the reaction zone at the position of maximum ray deflection as recorded in a schlieren image. Further simulations showed that the effects of exhaust gas recirculation and air-to-fuel ratio on the schlieren images were minimal under engine conditions compared to the temperature effect. At 20 crank angle degrees before top dead centre (i.e. 20 crank angle degrees after ignition timing), the measured temperature of the flame front was in agreement with the simulations (730-1320 K 2 depending on the shape of the flame front). Furthermore, the schlieren images identified the presence of hot gases ahead of the reaction zone due to diffusion and showed that there were no hot spots in the unburned mixture.

Analysis of tilted neutral planes for tall space fires with unsymmetrical openings using numerical simulation and Schlieren photography technique

“…Su and Bai (2016) demonstrated that the background-oriented schlieren technique can directly and immediately measure the neutral plane position of an internal fire whirl for a vertical shaft model (Su and Bai, 2016). The Schlieren technique uses the first derivative variation of the refractive index between the light and the flow field, as shown in equation (1) (Aleiferis et al , 2015; Kuehn and Goldstein, 2017): …”

Schlieren techniques for observations of long positive sparks: Review and application

“…To solve this problem, the high-order total variation regularisation can be introduced in the Abel inversion to reduce the influence of image noise. The quantitative analysis from Liu [78] reveals that for a spatial resolution of 50 μm/pixel, the error could be about 10% for temperature over 2000 K. The error can be reduced by improving the spatial resolution [79]. Therefore, a balance needs to be struck between the spatial resolution and FOV to achieve satisfactory accuracy for a specific observation target.…”

Temperature Measurement of Fluid Flows by Using a Focusing Schlieren Method

“…This technique has remarkable advantages like easy to implement, uses conventional light sources, is low cost and is very sensitive [2,3]. This method has been improved to face different types of problems in science and engineering [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. Even though initially it was used as a visualization method, it has been used to quantify temperature data in fluid flows [8,9,10,11,12,13,14,15,16].…”

“…Nowadays different variant of the optical system has been implemented such as color schlieren [10,11,12], background oriented schlieren [13,14] and calibration schlieren [15,16]. This method has allowed to measure simultaneously temperature and velocity in fluid flows [19,20,21,22].…”