Engine Efficiency and Power Density: Distinguishing Limits from Limitations

Transcription

1 Engine Efficiency and Power Density: Distinguishing Limits from Limitations Chris F. Edwards Advanced Energy Systems Laboratory Department of Mechanical Engineering Stanford University

2 Exergy to Engines Chemical Resource Restrained Reaction Unrestrained Reaction Electrostatic Work Batch Expansion Batch Expansion Flowing Expansion Flow Work Lorentz Work (MHD) Exergy Classification Architecture Engines Limits are imposed by the resource, environment, and physics governing transfers and transformations. Limitations are introduced by the choice of devices and processes i.e., by the architecture of an engine.

3 Efficiency, Effective Compression Ratio, and Ideal Architectures 100 First Law Efficiency (%) RCCI gasoline + Diesel, Gross-Indicated PCI gasoline, Gross-Indicated FPEC Diesel, Gross-Indicated Effective Compression Ratio (ρ/ρ 0 ) Fuel-air Atkinson cycle Fuel-air Otto cycle 70-80% of Otto cycle Jaguar AV133, 5.0 L DISI gasoline, ULEV2 Volvo TG103/G10A, 11.8 L SI natural gas Cummins 6BT5.9-G6, 5.9 L Diesel, turbocharged, Tier 1 Cummins QSM11-G4, 10.8 L Diesel, turbocharged, Tier 3 Volvo Penta TAD734GE, 7.2 L Diesel, turbocharged, Tier % Ideal Work, 60 75% Peak Pres., 75 90% Peak Temp.

4 Equivalence and Compression Ratios Efficiency and peak pressure require use of significant compression with a dilute mixture.

5 Use of Low Temperature Combustion Use of LTC to control NOx emissions limits work output to 6 7 bar IMEP (5 6 bar BMEP).

6 Atkinson? Not LTC? Efficient, pressure limited, high output operation might be achievable with optimal expansion and non dilute mixtures.

7 Spanning Exergy to Engines Chemical Resource Restrained Reaction Unrestrained Reaction Electrostatic Work Batch Expansion Batch Expansion Flowing Expansion Flow Work Lorentz Work (MHD) Exergy Classification Architecture Engines Limits are imposed by the resource, environment, and physics governing transfers and transformations. Limitations are introduced by the choice of devices and processes i.e., by the architecture of an engine.

8 Van Blarigan/Aichlmayr Free Piston Engine Concept

9 Free Piston Architecture for High CR Combustion chamber Gas driver Gas driver Free pistons Balanced forces, no bearing loads Long stroke to bore ratio for low surface to volume ratio Short residence time at min. V Can use linear alternator for work extraction (van Blarigan/Aichlmayr) Volume (V/V 0 ) Free-piston experiment Slider-crank Time (ms) 9

10 Stanford Free Piston, Extreme Compression Apparatus HC analyzer Heated sample line Band heaters Vacuum pump Sample bag Condenser 10

11 Diesel Combustion at High Compression CR = 30:1, 1050 K CR = 100:1, 1550 K #2 Diesel, 1 ms injection, EOI at TDC

12 Initial Diesel Efficiency Results Diesel #2 φ = Efficiency(%) Ideal 1 st -law efficiency Ideal cycle minus air experiment losses Experimental indicated efficiency Compression Ratio

13 Limited to Diesel Style Combustion? Premixed combustion is sootless. Premixed lean combustion is very efficient. Premixed stoich combustion has high power density. Premixed stoich combustion permits use of a TWC, and therefore very low NO x emissions. To accomplish this at high CR, autoignition must be held off until the minimum volume. Might be able to hold off autoignition by: Choice of fuel (e.g., methane/ng, methanol) Active cooling of the charge

14 Temperature Control of Autoignition T (K) T start = 298 K T start = 250 K Model: Adiabatic compression Homogeneous, stoichiometric methane air charge Volume time profile from experimental data GRI 3.0 chemical kinetics ρ/ρ 0 Lowering the initial gas temperature by 50 K lowers the temperature at 100:1 by 210 K. Ignition occurs at the desired volume.

15 Two Methods of Cooling T (K) S P = 1 atm 1 S Cooling air Compressor J T valve Products Refrigerant Engine ρ/ρ 0 1 Reactants at T 0, P 0 2 T (K) S S 3 P = 2.17 atm ρ/ρ 0 This is a common turbocharger / intercooler, Cooling air with 2.17 atm manifold pressure. Compressor 1 Reactants at T 0, P 0 2 Products 3 Engine

16 Pressure (bar) Experiments w/intercooling Volume / Full Cyl. Volume Fuel: premixed methane air Effective CR: 35 to 90:1 Equivalence ratio: 0.96 to 1.04 Experimental method: Charge compressed part way, remaining at wall T Usual rapid compression starts from that point Intercooling P chosen for ignition just after TDC Peak Efficiency: 57% (Includes comp. work.)

17 Measured Combustion Efficiency

18 NO x, HC, and CO Premixed Emissions w/intercooling Specific Emission (g/kw-hr) ~ 35:1 CR ~ 60:1 ~ 80:1 NO x Equivalence Ratio Specific Emission (g/kw-hr) HC Equivalence Ratio ~ 35:1 CR ~ 60:1 ~ 80:1 Specific Emission (g/kw-hr) CO ~ 35:1 CR ~ 60:1 ~ 80: Equivalence Ratio

19 NOx Emissions vs. GRI3.0 ~1% Loss in Combustion Efficiency 60:1 CR, methane

20 Emissions in Context of TWC 60:1 CR, φ 1 J. Chiu, J. Wegrzyn, and K. Murphy, SAE Paper I. Saanum, M. Bysveen, P. Tunestal, and B. Johansson, SAE Paper No

21 Evaporative Cooling? P (bar) % mass fraction total water injected V/V 0 Start water injection No cooling Water injection Model: Same as before, but with water vaporization Vaporization rate matches injection rate of real injector Start of injection chosen to avoid gas saturation Inject liquid during compression (water has good properties). Vaporization draws sensible energy from the gas, thus lowering the temperature. 21

22 Experiments w/evaporative Cooling Pressure (bar) Experiment EOI V/V 0 Model EOI Water injection Intercooling Water model for equal cooling SOI Achieved TDC phasing up to 60:1 CR More water needed (8% vs. 1%) 10% decrease in efficiency (53%) Limited by the injector setup: Stratification Slow vaporization 22

23 Reduction in Rate of Rise, Ringing Pressure (bar) Intercooling Water injection Maximum rate of pressure rise, translated to slidercrank at 1800 RPM: ~ 5000 bar/cad for intercooling approach ~ 80 bar/cad for water injection approach Time (ms)

24 Take Away Messages Exergy sets an absolute limit for the work from a resource in a specified set of surroundings. If you are not aspiring to approach this limit, please adjust your thinking. (Suspension of disbelief!) The physics of the various energy transfer and transformation processes that can be invoked sets additional limits. Take these seriously and change the processes used if necessary. The architecture you choose for your engine introduces limitations based on both the processes involved and the devices used to implement them. Track the exergy destruction through these devices to know how well you are doing. If you are not doing well (exergy efficiency below 50%), consider changing the set of processes, as well as improving the devices. Also consider using non traditional devices to implement the processes. The key to improvement is to know where you stand. (Absolutely!)