FILE ENGLISH Torque and power output Torque The conrod converts the linear motion of the piston into rotational motion of the crankshaft. The force with which the expanding air/fuel mixture forces the piston downwards is thus translated into rotational force or torque by the leverage of the crankshaft. The output torque M of the engine is, therefore, dependent on mean pressure pe (mean piston or operating pressure). It is expressed by the equation:M = pe . VH /(4.).WhereV¬H is the cubic capacity of the engine and. The mean pressure can reach levels of 8…22 bar in small turbocharged diesel engines for cars. By comparison, gasonline engines achieve levels of 7…11 bar. The maximum achievable torque, Mmax, that the engine can deliver is determind by its design (cubic capacity, method of aspiration, etc…). The torque output is adjusted to the requirements of the driving situation essentially by altering the fuel and air mass and mixing ratio. Torque increasses in relation to engine speed, n, until maximum torque, Mmax, is reached (Fig.1). As the engine speed increases beyond that point, the torque begins to fall again ( maximum permissible engine load, desired performance, gearbox design ). Engine design efforts are aimed at generating maximum torque at low engine speeds ( under 2,000 rpm ) because at those speeds fule consumption is at its most economical and the engine’s response characteristics are perceived as positive ( good “ pulling power”). Power output The power P ( work per unit of time) generated by the engine depends on torque M and engine speed n. Engine power output increases with engine speed unitl it reaches its maximum level, or rated power Prated at the engine’s rated speed, nrated. The following equation applies: P = 2 . . n . M Figure 1.a shows a comparison between the power curves of diesel engines made in 1968 and in 1998 in relation to engine speed. Due to their lower maximum engnine speeds, diesel engines have a lower displacement-related power output than gasonline engines. Modern diesel engines for cars have rated speeds of between 3,500 and 5,00000 rpm. Fig 1: Torque and power curves for two diesel car engines with a capacity of approx. Power curve Torque curve 1968 Engine 1998 EngineMmax Maximum forquesPrated Rated powernrated Rated speed Engine efficiencyThe internal-combustion engine does work by changing the pressure and volume of a working gas ( cylinder charge). Effective efficiency of the engine is the ratio between input engrgy (fuel) and useful work. This results from the thermal efficiency of an ideal work process ( Seiliger process) and the percentage losses of a real process. Seiliger process Reference can be made to the Seiliger process as a thermodynamic comparison process for the reciprocating-piston engine. It describes the theoretically useful work under ideal conditions. This ideal process assumes the following simplifications: Ideal gas as working medium Gas with constant specific heat No flow losses during gas exchangeFig. 1: Seiliger process for diesel engines 1-2 Isentropic compression 2-3 Isochoric heat propagation 3-3’ Isobaric heat propagation 3’-4 Isentropic expansion 4-1 Isochoric heat dissipationTDC Top dead center BDC Bottom dead centerqA Quantity of heat dissipated during gas exchangeqBp Combustion heat at constant pressureqBV Combustion heart at constant volumeW Theoretical work The state of the working gas can be described by specifying ( p) and volume (V). Changes in state are presented in the p-V chart (Fig.1), where the enclosed area corre-sponds to work that is carried out in an operating cycle. In the Seiliger process, the following process steps take place:Isentropic compression (1-2) With isentropic comopression ( compression at constant entropy, i.e.without transfer of heat), pressure in the cylinder increases while the volume of the gas decreases. Isochirc heat propagation (2-3) The air/fuel mixture starts to burn. Heart propagation (qBV) takes place at a constant volume (isochoric). Gas pressure also increases. Isobaric heat propagation (3-3’) Further heat propagation (qBp) takes place at constant pressure ( isobaric) as the piston moves downwards and gas volume increases. Isentropic expansion (3’-4) The piston continuses to move downwards to bottom dead center. No further heat transfer takes place. Pressure drops as volume increases. Isochoric heat dissipation (4-1) During the gas-exchange phase, the remaining heat is removed (qA). This takes phace at a constant gas volume ( completely and at infinte speed). The initial situation is thus restored and a new operating cycle begins.p-V chart of the real process To determine the work done in the real process, the pressure curve in the cylinder is measured and presented in the p-V chart ( Fig. 2 ). The area of upper curve corresponds to the work present at the piston.Fig. 2: Real process in a turocharged/supercharged diesel engine represented by p-V indicator diagram EO Exhaust opens EC Exhaust closes SOC Start of combustion IO Inlet opens IC Inlel closes TDC Top dead center BDC Bottom dead center pU Ambient pressure pL Charge-air pressure pZ Maximum cylinder pressure Vc Compression volume Vh Swept volume WM Indexed work WG Work during gas exchange ( turbocharger/supercharger) Fig. 3: Pressure vs. crankshaft rotation curve (p-a diagram) for a turbocharge/supercharged diesel engine EO Exhaust opens EC Exhaust closes SOC Start of combustion IO Inlet opens IC Inlet closes TDC Top dead center BDC Bottom dead center pV Ambient pressure pL Charge-air pressure pZ Maximum cylinder pressure For assisted-aspiration engines, the gas-ex-change area (WG) has to be added to this since the compressed air delivered by the turbocharger/supercharger also helps to press the piston downwards on the induction stroke. Losses caused by exchange are over-compensated at many operating points by the supercharger/turbocharger, resulting in a positive contribution to the work done.
Representation of pressure by means of the crankshaft angle (Fig. 3, previous page) is used in the thrmodynamic pressure-curve analyis, for example.
Efficiency
Effictive efficiency of the diesel engine is defined as: ne = We / WB
We is the work effectively available at the crankshaft.
WB is the calorific value of the fuel supplied
Effective efficiency ne is representable as the product of the thermal efficiency of the ideal process and other efficiencies that include the influences of the real process:
ne = nth . ng . nb . nm = ni . nm
where
nth : thermal efficiency
nth is the thermal efficiency of the Seiliger process. This process considers heat losses occurring in the ideal process and is mainly dependent on compression ratio and excess-air factor.
As the diesel run at a higher compression ratio than a gasonline engine and high excess-air factor, it achieves higher efficiency.
ng : efficiency of cycle factor
ng specifies work done in the real high-pressure work process as a factor of the thearetical work of the Seiliger process.
Deviations between the real and the ideal processes mainly result from use of the real working gas, the finite velocity of heat propagation and dissipation, the position of heat propagation, wall heat loss, and flow losses during the gas-exchange process.
nb : fuel conversion factor
nb considers losses occurring due to incomplete fuel combustion in the sylinder.
nm : mechanical effciency
nm includers friction losses and losses arising from driving ancillary assemblies. Frictional and power-tranmission losses increase with engine speed. At nominal speed, frictional losses are composed of the following:
+ Pistons and piston rings approx. 50%
+ Bearing approx. 20%
+ Oil pump approx. 10%
+ Coolant pump approx. 5%
+ Valve-gear approx. 10%
+ Fule-injection pump approx. 5%
If the engine has a supercharger, this must also be included.
ni : efficiency index
The efficiency index is the ratio between ‘indexed’ work present at the piston Wi and the calorific value of the fuel supplied.
Work effectively avilable at the crankshaft We results from indexed work taking mechanical losses into consideration:
We = Wi . nm
Operating statuses
Starting
Staring an engine involves the following stages: cranking, ignition and running up to self-sustained operation.
The hot, compressed air produced by the compression stroke has to ignite the injected fuel (combustion start). The minimum ignition temperature requierd for diesel fule is approx. 250oC.
This temperature must also be reached in poor conditions. Low engine speeds, low outside temperature (Fig. 1: Compression pressure and ultimate temperature relative to engine speed). The reasons for this phenomenon are leakage losses through the piston ring gaps between the piston and the cylinder wall and the fact that when the engine is first started, there is no thermal expansion and an oil film has not formed. Due to heat loss during compression, maximum compression temperature is reached a few degrees before TDC (thermodynamic loss angle, Fig. 2)
Fig. 2: Compression pressure as a factor of crackshaft angle
+ fa Outside temperature
+ tZ Ignition temperature of diesel fuel
+ aT Thermodynamic loss angle
+ n = 200 rpm
When the engine is cold, heat loss occurs across the combustion-chamber surface area during the compression stroke. On indirect-injection (IDI) engines, this heat loss is particularly high due to the larger surface area.
Internal engine friction is higher viscosity of the engine oil. For this reason, and also due to low battery voltage, the starter-motor speed is only relatively low.
The speed of the starter motor is particularly low when it is cold because the battery voltage drops at low temperatures.
The following measu
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