INFLUENCE OF BRAKING VALVE OPENING AND CLOSING ANGLES UPON TORQUE

Engine brake systems are based on the concept of spending a large amount of the vehicle’s kinetic energy by releasing pressure from the cylinder. Although an important part of the kinetic energy is used during compression, unless the spring effect of gases is suppressed, a good portion of it is returned to the wheel during the power stroke. This goal can be achieved by releasing of compression just before maximum cylinder pressure is reached. The present paper illustrates investigations upon the exhaust valve opening and closing angles that determine a braking event and their influence upon engine torque.


INTRODUCTION
In the 60's, Cummins developed the concept of engine brake by aid of releasing compression.The transition from normal functioning (with combustion) to braking mode is described in [1].Compression release is achieved by opening the exhaust valve.Further on, the exhaust valve that opens in order to generate the engine brake event is called brake valve.By opening of the brake valve, the engine is turned into a giant compressor that consumes part of the crankshaft kinetic energy.The kinetic energy absorbed from the crankshaft is released as heat during the braking process.The amount of kinetic energy spent during an operating cycle is directly proportional to nominal torque.
The transition between the two engine operating modes, the conventional one and the engine brake, respectively takes place by total suppression of injection and opening of the brake valve near the top dead center during the compression stroke.The easiest way to activate the engine brake regime is to transfer the injector attack force the injector to the operating mechanism of the brake valve.This operating procedure ensures that the brake valve's opening height; opening timing and closing delay are invariable with speed.
In order to determine pressure forces generated by gases present inside the cylinder, it was first assumed that crankcase gases are under constant pressure, at 1 bar.The cylinder gas pressures matrix was obtained using a mathematical model previously developed by the authors [5] for the assessment of thermo-gas-dynamic parameters of a compression ignition engine.Although the abovementioned model allows computation of cylinder pressure corresponding to all engine processes, both under normal operating conditions and under braking regime, only the sub-matrix of resulted pressures corresponding to compression, combustion or braking and relaxation is of interest.The P-diagrams corresponding to compression, combustion, engine brake and relaxation, respectively, plotted in Figure 1, were obtained using five brake valve opening crank angles, which generated as many sub-matrices containing pressure values.Based on injection timing, the initial value of the crankshaft angle corresponding to brake valve opening was chosen 5° .Similarly, the initial brake valve closing delay is = 22° , chosen in relation to the end of combustion.The maximum height of the brake valve lift is chosen to be smaller than the combustion chamber length (l ca ), determined as . Thus, the maximum lift of the brake valve was adopted at 0.8 mm, constant value in the presented study.The brake valve raising law is given in matrix form, generated in Mathcad by aid of a cubic spline interpolation.
In order to evaluate engine torque, gas pressure generated forces must be determined, by aid of: Determination of translating inertia forces is conducted by splitting the connecting rod weight in two components: one solidary to the translation motions of the piston (m bp ), while the other rotates simultaneously with the crankshaft.Thus, the translation inertia force is given by the product between piston acceleration and the combined weights of the piston and associated connecting rod.The acceleration (a p ) can be written as a function of length ratio of crank to connecting rod ( b ), as follows: The inertia force (Ft j ) can be determined by summation of these two components and has the following expression: where: mt j is the combined weights of the piston and associated connecting rod, S -stroke, n -speed,crankshaft rotation angle, b -length ratio of crank to connecting rod.
Using the load bearing weights of the piston and connecting rod from [4] as input data, inertia forces and its first and second order harmonics were plotted in Figure 2. Fig. 2. Inertia force along with its first and second order harmonics.
The resulting force (Fb), applied by the piston to the crankshaft via connecting rod, is determined by summation of equations ( 1) and (5).As only the force component oriented along the connecting rod axis has an effect on the torque generated by the crankshaft, Fb is splited along two directions.The expression of this force is: In a similar manner, only the tangential component of connecting rod-crankshaft joint force generates torque and can be calculated at the crankpin by:

sin [] cos
Tg a zj where is the angular displacement of the connecting rod.
Once the forces acting on the motor mechanism are determined, the available flywheel torque can be evaluated using the general relation for momentum.It should be noted that friction and other auxiliary forces induced by connected systems are neglected, as only the evolution of torque during engine brake regime is of interest and not its actual value.Due to the fact that the present investigations are conducted considering a single cylinder engine, the momentary torque can be determined as: The torque variation diagram is represented in Figure 3 for normal operating regime (with combustion).

INFLUENCE OF LIMITING CRANK ANGLES ON ENGINE BRAKE PROCESS
The use of engine braking system is characteristic to freight and passenger transport vehicles.During this operating regime, the average torque per cycle must be at a minimum.This ensures maximum consumption of the crankshaft kinetic energy.For the present study, instantaneous torque values produced by crankpin tangential forces are calculated for five values of the brake valve opening timing.
The first value was chosen at 5° , due to the fact that the brake process is easily induced to the engine cycle by simply transferring the injector command to the brake valve mechanism.The other four values are chosen equidistant and symmetric to the initial crank angle value.Thus, the brake valve opening angles corresponding to the five analyzed cases are: = (340, 345, 355, 360, 365).The braking process duration and the valve lifting law were kept constant in all five cases.The variation of available torque corresponding to the flywheel was plotted against crank angle for all five considered situations, as shown in Figure 4.The diagram presented in Figure 4 illustrates that for the initial crank angle value of 355 o RA, the torque shows an increase after passing the top dead center, which diminishes the braking effect.By increasing the crank angle corresponding to opening of brake valve, the momentary torque maintains its increasing tendency past the top dead center.This increase is due to high cylinder pressures, which consequently generate a significant spring effect from the gasses.
By choosing angles below 355 o RA for the brake valve opening angle, a decrease in torque just after the top dead center can be noticed.However, the obtained results illustrate that for brake valve opening angles below a certain value, the average torque over the investigated domain increases.This increase, disadvantageous for the braking process is present mostly because of the diminishing of braking momentum before reaching the top dead center, which in its turn is due to low gas pressures inside the cylinder at the moment of brake valve opening.
From the results illustrated by Figure 4, it can be concluded that for the particular braking process duration and engine type considered, the optimum timing for brake valve opening is situated in the vicinity of 5 o RA.
By introducing the momentary torque matrix in Microsoft Excel, the diagrams shown in Figure 4 were analyzed using planimetry, which allows for graphic integration of torque curves by aid of surface measurements.The planimetric analysis was conducted using the method of rectangles.The yielded values represent the mean torque corresponding to each particular study case.The diagrams plotted in Figure 5 illustrate a comparison between torques obtained for the best case braking regime and those corresponding to normal operating conditions.
Extreme values of torque determined in the immediate proximity of the top dead center, as well as mean torques corresponding to the five investigated situations are summarized in Table 1.

CONCLUSIONS
The work presented herein can be summarized by the following conclusions: -engine generated torque diminishes with the increase of brake valve opening advance, until a certain limit is reached; -as the brake valve opening angle is closer to the top dead center, the mean torque increases, which is detrimental to braking process efficiency; -the most efficient of the investigated cases was obtained when the braking valve opens with an advance of 5 o RA by report to the top dead center.In this case, the braking process generates a mean negative torque of approximately 40 % from the mean torque developed by the engine under normal operating conditions (with combustion).