Engine Testing: Efficiency, Air Fuel Ratio, and Volumetric Efficiency, Lecture notes of Design

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Power Flow and EfficiencyPower Flow and Efficiency

Efficiencies When the engine converts fuel into power, the process is rather inefficient andonly about a quarter of the potential energy in the fuel is released as power atthe flywheel.The rest is wasted as heat going down the exhaust and into the air or water. Thisratio of actual to potential power is called the "THERMAL EFFICIENCY", ofthe engine.How much energy reaches the flywheel ( or dynamometer) compared to howmuch could theoretically be released is a function of three efficiencies, namely: 1. Thermal 2 Mechanical 3.^ Volumetric

B.S. = Brake Specific Brake Specific Fuel Consumption = mass flow rate of fuel

÷

power output

bsHC = Brake Specific Hydrocarbons = mass of hydrocarbons/power output.e.g. 0.21 kg /kW hour

Thermal Efficiency

Since, Q

HV

for petrol = 43.5 MJ/kg

Therefore, the brake thermal efficiency = 82.76/ sfcThe indicated thermal efficiency = 82.76 / isfc

Sfc

kg MJ Q hr kW g Sfc Q Sfc Therefore

P Sfc

rate flow

mass fuel

fuel of

value

heating Q

cycle per fuel of mass m

output

power P

cycle per

work W

P Q

Q W m

HV

HV

t

f s c s f HV f

HV s f

HV c f

t

(^76). 82 ) / ( ) / (

3600

1

n

consumptio fuel

Specific

_ _ _

_ _

_

_ _ _ _ _

_ _

=

⋅

= ⋅ − − − − − = =

=

= η

μ μ

μ

η

Volumetric Efficiency

The parameter used to measure an engine’s ability to breath air is the volumetricefficiency

e

temperatur and

pressure

ambient at

cylinder per

volume

swept

occupy to air of

mass

cycle per

cylinder per

indicated air of

mass

v^

---

_ _ _ _ _ _ _ = η

Swept Volume •^

At top dead centre, the volume remaining above the piston is termed theclearance volume. The swept volume is defined as the volume above the pistonat bottom dead centre, less the clearance volume.

-^

Hence

SWEPT VOLUME = Total volume - clearance volume.

Compression ratio C R

Typical values for compression ratio are 8 to 12:1 for spark ignition enginesand 12 to 24:1 for compression ignition engines.

volume

clearance

volume

clearance

me

swept volu

1 2

V V

Brake Specific Air Consumption (bsac)Similarly, specific air consumption is defined as the air massflow rate per unit power output:^ Bsac = Air flow

÷

Power

IMEP Indicated Mean Effective Pressure IMEP = Indicated work out put per cylinder per mechanical cycle/

Swept volume per cylinder

PMEP Pumping Mean Effective Pressure This is a measure of the work done in drawing a fresh mixture through the inductionsystem into the cylinder, and to expel the burnt gases out of the cylinder and throughthe exhaust system. CEMEP Compression/Expansion Mean Effective Pressure -the same as gross IMEP FMEP Friction Mean Effective pressure This is a measure of rubbing friction work and accessory work

Efficiency – comments 1 •^

The efficiency of an internal combustion engine depends on where and whenin the cycle the heat is released. In an ideal Otto cycle (Four-stroke cycle) thismeans that heat release must occur at a constant volume. This however is notpossible as combustion takes a finite time.

-^

As the piston approaches the top of it’s stroke it slows, momentarily comes torest and then moves back down. Ideally heat release (Combustion) should takeplace as close to this point as possible, as any heat release that occurs beforethis point causes a pressure increase that opposes the pistons last upwardmovement, and the opposition to the pistons movement represents a loss inefficiency.

Efficiency – comments 3 •

In theory this ideal can be achieved by offsetting the crankshaft axis from thepiston axis.

-^

The additional effect of offsetting the crank is that the combination of thealtered time/pressure history in the cylinder and the altered moment of theforce acting through the connecting rod change the instantaneous torque at thecrankshaft.

-^

An elegant solution to this is the Australian Scotch Yoke mechanism patentedand manufactured with VW by the Collins Motor corporation of Australia

-^

The efficiency is fairly fixed for a given design of engine. It is generally basedon the factors effecting heat loss, such as the design of the combustionchamber and materials used, and the compression ratio. The efficiency doesnot vary dramatically and tends to range from .34 to..

-^

Due to there close relationship the indicated and combustion efficiencies areoften evolved together as a fuel conversion efficiency,

ηf

Mechanical efficiency losses^ •^

Friction losses in the engine come from friction between moving parts (suchas the piston rings on the cylinder walls), the power required to run the valvegear and the pumps and the pumping losses involved in getting the gases inand out of the cylinder. A large part of the work appears as heat in thecoolant and oil. The approximate breakdown of the contributions of each ofthe losses is as follows: Friction losses: •^

Crankshaft and seals – 12%

-^

Pistons, rings, pins and rods – 46%

-^

Valve train –23%

-^

Oil pump – 6%

-^

Water pump and Alternator – 13%

The mass flow rate of air^ : Control and optimisation of the three variables engine speed (N), volumetricefficiency (

ηv

) and Inlet density is very important and can potentially allow the

tapping of extra energy from the bsfc.

Volumetric efficiency

If we think of the engine as an air pump then theoretically it should draw inand exhaust its own volume of air each time it cycles - that is, once everyrevolution if it's a two stroke and once every two revolutions if its a fourstroke.In fact, ordinary production engines don't achieve this and only manage toshift about 80% of their volume.This ratio of possible air pumped to actual air pumped is called VolumetricEfficiency and this is what we have to improve to get more power.