Variable Compreesion Ratio

7/31/2019 Variable Compreesion Ratio

1/21

Introduction

Compression Ratio (CR) is the ratio of total (swept + clearance) volume to theclearance volume

CR of traditional engine is fixed and is a compromise across a wide range ofoperating conditions

The main feature of the VCR engine is to operate at different compression ratios,depending on the vehicle performance needs .A VCR engine can continuously vary

the compression ratio by changing the combustion chamber volume At low power levels, the VCR engine operates at a higher compression ratio to

capture high fuel efficiency benefits, while at high power levels the engineoperates at low compression ratio to prevent knock

The optimum compression ratio is determined as a function of one or morevehicle operating parameters such as inlet air temperature, engine coolanttemperature, exhaust gas temperature, engine knock, fuel type, octane rating offuel, etc.

It has been proven that a VCR engine develops much more power for the sameengine dimensions, i.e. it is very compact and has a high power-to-weight ratio

without any penalty on specific fuel consumption


2/21

Various VCR approaches

Moving the crankshaft axis

Modification of the connecting rod geometry

Moving the cylinder head

Variation of combustion chamber volume using asecondary piston or valve

Variation of piston deck height

Moving the crankpins


3/21


4/21

Otto Cycle At Part Load

Analyzing the work involved in this cycle

The thermal efficiency is calculated as:


5/21

Otto Cycle At Part Load

Knowing that the maximum work produced by theOtto cycle is for the wide open throttle (WOT)operation

Wmax = Wwhenp1 = p0 or = 1


6/21


7/21

Otto Cycle Variable Compression Engine

Increasing the intake pressure ratio (), defined as the ratio between the intakepressure (p1) and the atmospheric pressure, and keeping temperature (andpressure) level of points 2 and 3, as high as possible, the compression ratio willhave to be adjusted (reduced) to prevent knock onset


8/21

Otto Cycle Variable Compression Engine

The variation of is shown to decrease the thermal efficiency of the engine withthe reduction of the load. In that case it is also considered that the compressionratio is constant.

If that process would be described as isentropic a different behavior of theefficiency would be obtained for different throttling conditions. Also if the

compression ratio could be adjusted to the intake pressure in order to get thesame pressure (p2limit) at the end of compression


9/21


10/21

Zero Dimensional Model

To find cylinder pressure, heat release rate, brakethermal efficiency

Prediction of heat release and gas properties

The rate of heat release from the cylinder

walls


11/21

Cylinder Volume v/s Crank Angle

In cylinder volume at each crank angle position

Instantaneous cylinder volume is a function of compression ratio. Thus varying the compression ratio will affect the cylinder volume as afunction of crank angle. Rate of change of cylinder volume with crank angle is further used in

calculation of temperature and pressure with crank angle. Pressure crank angle diagram is used for analysis of further parameters


12/21

Rate Of Heat Release

The rate of heat release has been estimated fromWiebes heat releasemodel

Gas properties during the compression and expansion have beencalculated using isentropic relations


13/21

Ignition Delay & Engine Performance Parameters

Calculation Of Ignition Delay:

Hardenbergs Model

Indicated Power and Brake Power calculations Frictional Power Calculations1. Mean effective pressure lost to

overcome friction due to gaspressure behind the rings

2. Mean effective pressure

absorbed in friction due to walltension of rings3. Mean effective pressure

absorbed in friction due topiston and rings


14/21

Multi Fuel Capability of a Homogeneous Charge Compression Ignition Enginewith Variable Compression Ratio


15/21

Cylinder Pressure and Rate of Combustion

Compression Ratio

Inlet Temperature

Octane Number


16/21

Cylinder Pressure and Rate of Combustion


17/21

Gross and Net Indicated Efficiency

Gross Indicated Efficiency

Net Indicated Efficiency


18/21

Gross Indicated Thermal Efficiency

Gross Indicated Thermal Efficiency


19/21

Challenges in achieving VCR

Major changes in engine base design and hencebarrier to widespread adoption of technology

Compromise ideal geometry and layout of the

valves and ports, due to additional elements Additional crevice volumes, High HC emissions

Increase in reciprocating mass

Increase in vibrations

Consists of multilink rod-crank mechanisms


20/21

Conclusions

Optimum combustion efficiency in the whole load and speedrange

Low fuel consumption and low exhaust emissions;

High fuel flexibility with optimal combustion efficiency;

Very smooth idle and full load accelerations are achieved; Provides better indicated thermal efficiency than that of FCR

engines

Allows for a significant idle speed reduction because ofreduced misfiring and cyclic irregularities, resulting in low

vibration levels Reduction in low-frequency noise because of constant peak

pressure


21/21

Variable Compreesion Ratio

Documents

Transcript of Variable Compreesion Ratio