Efficiency of Spark Ignition, Gasoline Engines

First, one must look at the monetary return for the effort. When gas is consistantly over $4.00/Gallon USD you will see amazing strides in technology. Until then, forget it.

For starters there are many different aspects of a vehicle that determine its overall fuel efficiency. Aerodynamic drag and combustion efficiency are but two of many.

Making a vehicle lighter reduces rolling friction which does have a slight gain for fuel efficiency but the down side is when a vehicle becomes too light it becomes unstable and dangerous on the road at some point. A one or two MPG fuel gain by weight reduction could be the tipping point where the vehicle becomes too light to be stable at road speeds and very unstable in cross winds or in less than ideal weather, traction, or loaded conditions.

Making a vehicle super aerodynamic may also give a gain but then at some point the vehicle looks more like a pointy stick and is uncomfortable or becomes to hard to get in and out of or simply becomes too impractical for the average person to use. Its basically too aerodynamic to be functional for the average driver and what they need it for.

Powertrain wise there are numerous ways to put the power to the ground but at some point that pursuit of ultimate efficiency creates a drive train that is too expensive, high in maintenance, or just to complicated to be practical. Thats what you see with most of the hybrid designs today. They offer only slight gains in everyday driving but push the complexity and cost up considerably which usually does not justify the added costs for their design when compared to what little long term gain was achieved over the long run.

The engine itself has a whole load of different aspects that determine its efficiency not just the ideal thermodynamic cycles. At one time there where big heavy aerodynamically challenged steel monsters with basic drive trains that where all moved by massive engines that when maintained got around the same fuel economy as the new vehicles of today. The difference was politics not mechanics, physics or chemistry that changed the efficiency of engines. Emissions regulations put in place by clueless half wits are what severely reduced the efficiency of engines not the fuel or the mechanics.

These are just a few of the basic things that affect how much fuel a vehicle of any size weight or design uses in its daily operations. Mechanics and physics determine the real efficiency while politics and ill conceived beliefs held by people not properly educated in the real sciences are what continue to create the worst efficiency losses in vehicle design.

All the designing in the world is still useless if politics dictates that the one key component that makes a vehicle do what it does is required to work at far less than its peak efficiency just to meet some ill conceived notion that by burning more fuel less efficiently you are emitting less combustion byproducts.

"Emissions regulations put in place by clueless half wits are what severely reduced the efficiency of engines not the fuel or the mechanics. ... ..some ill conceived notion that by burning more fuel less efficiently you are emitting less combustion byproducts."

Can you explain how reducing emissions of pollutants (HC, CO & NOx) we end up with less efficient cars? Is this linked to sustaining reaction temps in the catalytic converters?

Thanks, TCMTECH!

Beautiful stuff. I have been riding around with catalytic converters for many years, and had never thought about what they were doing and how they were doing it.

I appreciate your contribution to this conversation.

Do you know of any EPA or DOT or other agency performing measurements of the efficiency of automotive gas engines? Is there a listing of these measurements somewhere?

Hamish

Can the quenching effect be reproduced to some decree by stratifying the charge whilst still running at stoichiometry (or leaner)? I think I get the idea of allowing a little of the fuel to only partially burn (boosting COs & HCs) in return for absorbing more of the heat energy into mechanical work (through expansion) - I just was under the impression that with modern combustion control a-best-of-both-worlds solution was (at least in theory) possible, ie. all the fuel is completely burnt (in strata - with lean regions between), so the expansion is a modular thing, adding up to a smoother (hence more efficient) delivery of the chemical energy to expansion.

You have some things about fuel air mix in gasoline engines confused here.

Most gasoline engines today run a rich fuel air mix... the number of gasoline engines burning a stoikiometric or even a lean fuel air mix are a small but growing number limited to very high efficiency models. While what you say about the emissions cam (and the beaurocrats) is absolutely correct, the cam would have essultially nothing unburnt to wastefully bleed from the cylinder if a balanced or lean fuel air mix were used. Stoikiometrically balanced as well as lean mixes typically detonate, only rich mixes burn slowly enough for the reaction to still be occuring when the exhaust valves first open.

Additionally, most gasoline engines, are not designed to handle detonation, and take great lengths to avoid it..including running rich.

Greater efficiencies are available using lean burn mixes, but satisfying the emmissions reglations is trickier when esseltially all the fuel is detonated/reacted in the cylinder and none is wasted on heating the exhaust.

check out www.fueleconomy.gov for numerous examples of emmissions standards killing efficiency and causing excess fuel to be used just to meet bureacrats arbitrary ratios..

Bbb.....flee to me remote elf.

Apologies if I've made an assumption here, but when you say "engines burning a stoichiometric or even a lean fuel air mix are a small but growing number limited to very high efficiency models", is this an North-America-centric statement or a wider 'truth'?

I don't think it's valid to say that lean-burn leads to detonation (which is certainly to be avoided in S.I. engines). Lean-burn modes can comfortably be sustained, provided the engine is designed for such a mode, plus a high enough octane fuel is used and it is not faulty in some other way.

I also think that it is lazy for manufactures to use the premise of keeping the cat heated as an excuse for poorer FE. If heat is the driving factor for a cat' then there is still going to be enough heat 'thrown out' in the exhaust stream (with thoughtful design of the manifold & cat pathway) - even with a continual lean-burn operation.

Smaller (or equivalent HP hybrid) engines will require smaller cats also, and so should need smaller quantities of hot gas throughput. This leads me to ask whether it would ever be advantageous (particularly with hybrids) to boost the cats internal temp electrically?

" when a vehicle becomes too light it becomes unstable and dangerous on the road at some point. "

Have you checked on the power and weight of the cars in Formula 1, Indy Car Series or the older Can Am series. It seems no one told them that their cars are too light.

All racing vehicles are either designed or modified to look like an inverted wing to the air it runs through. Some even have an adjustable wing ie: Formula 1. The purpose is to create a downward force onto the wheels giving the traction needed at speeds for stability. This way they can have the proper weight for the speed at hand.

Hummm... adjustable weight.

Take their air foils off and see how one handles in daily traffic while being driven by the average person! Or just come up to north Dakota and try driving one in one of our regular gusty 40+ MPH cross winds.

High performance race cars are not a good example of daily driver vehicles owned and operated by the average clueless dolts with one hand on the cell phone and the other someplace besides the steering wheel.

The issue was not driver quality. It was that vehicles become unstable when they become too light. Would you like to compare the stability of a non race Lotus to a 1958 Buick? It must be the extra weight that makes the Buick so agil on the twisted roads.

Okay... You got me on that one. How does 52+ year old sedan suspension and handling compare to that of a modern high performance custom niche market vehicles?

As I understood it the OP was asking about issues related modern vehicles that would most likely be of the typical standards that the average person owns and drives category not specialty race cars or grandpas classic Sunday driving sedans from over a half century ago.

I am getting from your examples seems to be a sort of like an apples to birds to bricks comparison spread out over five plus decade time frame.

tcmtech:"Although the 12.9:1 A/F ratio is richer in fuel it burns more efficiently due to a condition called fuel quench. This quenching effect along with a different camshaft design that favors the mechanical efficiency ratio over combustion cleanliness is very beneficial to converting that combustion energy into mechanical energy and not heat that goes out the tailpipe."

Can the quenching effect be reproduced to some decree by stratifying the charge whilst still running at stoichiometry (or leaner)? I think I get the idea of allowing a little of the fuel to only partially burn (boosting COs & HCs) in return for absorbing more of the heat energy into mechanical work (through expansion) - I just was under the impression that with modern combustion control a-best-of-both-worlds solution was (at least in theory) possible, ie. all the fuel is completely burnt (in strata - with lean regions between), so the expansion is a modular thing, adding up to a smoother (hence more efficient) delivery of the chemical energy to expansion.

You are thinking of it as digital condition where it all on or all the other that fortunately is not how it works.

The further you get from one and the closer you get to the other is variable. Half way in between is just that a roughly middle condition between the two. A little more power and better mechanical conversion efficiency and a little less theoretically ideal burn characteristics.

Slow down and hold on to your smilies there tcmtec. Reread my 2 cents and who I was applying it to. The racing circuit is there for three reasons, bragging rights, money, and to test future technology for the automotive industry. Not who has the biggest pretzel. Adjustable wings are starting to become a common sight on new vehicles. My aim was to inform on the true working weight of race cars.

In reality, I was standing behind your statements.

Has anyones ever tryd running the fuel between the pole faces of a neodyium magnet or adding a chemical to make it burn more efficiently or adding water to make steam.

I think these are good ideas and am looking for money to do more research.

You have to love British humor! These are classified ads, which were actually placed in a U.K. Newspaper: FREE YORKSHIRE TERRIER. 8 years old. Hateful little bastard. Bites! FREE PUPPIES. 1/2 Cocker Spaniel, 1/2 sneaky neighbour's dog. FREE PUPPIES. Mother is a Kennel Club registered German Shepherd. Father is a Super Dog, able to leap tall fences in a single bound. COWS, CALVES: NEVER BRED. Also 1 gay bull for sale. JOINING NUDIST COLONY! Must sell washer and dryer £100. WEDDING DRESS FOR SALE . Worn once by mistake. Call Stephanie. **** And the WINNER is... **** FOR SALE BY OWNER. Complete set of Encyclopaedia Britannica, 45 volumes. Excellent condition, £200 or best offer. No longer needed, got married, wife knows everything. Statement of the Century Thought from the Greatest Living Scottish Thinker. Billy Connolly - "If women are so bloody perfect at multitasking, how come they can't have a headache and sex at the same time?"

add volumetric afficiency to your list to ponder