Engine compression ratio - how does it affect performance and economy?

Discussion in 'Highly Technical' started by cheesehead, Jul 19, 2008.

  1. cheesehead

    cheesehead Lifer

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    In an internal combustion engine, a piston compresses a large volume of a mixture of fuel and air into a very small space. The ratio of the maximum piston volume to the minimum compressed volume is called the "compression ratio."

    Compressing the fuel and air will make them burn faster, which (though I'm not sure directly how) makes the engine run better. Due to the high compression ratio (12.51) of the 11,000 RPM Hayabusa engine and the low compression ratio (9.8:1) of the 6500rpm Mustang V8, I'm guessing that this allows for a much higher redline - the faster burn speed of the compressed fuel-air mix in the Hayabusa engine would allow it to complete burning before the piston had completed its stroke at high RPMs.

    There are secondary benefits to high compression ratios, too. High compression ratio engines burn both much more cleanly and much more efficiently than lower-compression engines. For example, a diesel engine, which burns fuel very differently to a gasolene engine, will often give fuel economy 60% greater than its gas equivalent, even though diesel only has about 10% more energy per gallon.

    According to Wikipedia, the increase in efficiency is due to the additional heat and brownian motion caused by compression fully vaporizing the fuel, which I think sounds a little fishy considering how much work is put into cooling the fuel-air mix in turbocharged cars. Most other websites say that it's due to the Carnot cycle, which I honestly do not understand - could someone explain it?

    Another issue is engine efficiency as a function of RPMs. An engine limits power by reducing the intake of fuel and air to an engine; if only half the fuel and air is entering a piston, the compression ratio is effectively halved as well.

    Considering all the advantages of high compression, one might wonder why anyone would not use a high compression ratio. The answer is simple: The increased heat density of the compressed gas will cause the fuel to begin combustion without ignition by the spark plug, resulting in an undesirable burn pattern. This detonation, or "knock", is often heard as a pinging noise and can cause severe damage to your engine.


    The measure of a fuel's minimum ignition temperature and resistance to detonation is its octane, which is not, as is commonly understood, a measure of it's energy per liter. Before the advent of the catalytic cracker, fuel was often below seventy octane, and engine compression ratios were low - a Model T had a compression ratio of 4.5 to 1. However, by splitting large molecules into smaller ones (cracking), modern engines are both more efficient and better performing than their older bretheren.

    Modern gas has an octane of about 93 for premium in the US, and about 97 in the rest of the world. 100+ octane gas can be had, but it's very expensive (well over $5/gallon) and often has octane-boosting additives which contain lead. However, all of these are dwarfed by ethanol, which as an octane rating of a whopping 129. As a result, it can easily be used in engines with a compression ratio of 15:1 or greater, and despite having an energy density only 2/3 that of gasolene, it should produce similar fuel economy in such an ethanol-optimized engines along with very, very high redlines.

    So, any thoughts on any of this?
     
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  3. KIAman

    KIAman Diamond Member

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    There is a direct relation to compression ratio and performance and efficiency (depending on engine construction and materials).

    There is also a direct relation between compression ratio and engine reliability (depending on engine construction and materials).

    There is a direct relation between engine construction and materials and cost.

    There is an intersection point between all 3 lines which provide optimal power, efficiency, compression ratio, reliability and cost.

    Also note that compression ratio is only a single factor in engine performance dynamics. There are many other cost effective engine performance enhancements, for example superchargers.

    There are cars readily available that take advantage of the power and efficiency of very high compression ratios (14/1+) but sacrifice on noise, vibration, cleanliness and fuel smell; diesel.
     
  4. frostedflakes

    frostedflakes Diamond Member

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    And something that should be noted is that noise, vibration, cleanliness, and fuel smell are not necessarily due to higher compression ratio. Most of those problems are flaws of the diesel cycle in general or the fuel (for example, the "rotten-egg" smell is caused by diesel fuels with high sulfur content). Also, more advanced diesel ignition systems, such as HCCI, promise to reduce NOx and particulate emissions for diesel engines.

    About ethanol, you are right, with an engine optimized for it very good efficiencies can be achieved. In the EPA study below they modified a diesel engine for spark ignition using neat and blended alcohol fuels and were able to achieve 40%+ efficiency. I believe this is on-par with a naturally-aspirated diesel engine and about 10% more efficient than even the best gasoline engines. Pretty interesting stuff.

    http://www.epa.gov/otaq/presen.../epa-fev-isaf-no55.pdf
     
  5. blahblah99

    blahblah99 Platinum Member

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    What's wrong with that article? Sounds valid to me.

    The higher the compression ratio, the higher the octane rating required to resist knock due to super-heated gas that results from compressing air+fuel.
     
  6. Vee

    Vee Senior member

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    Expansion ratio!

    A thermodynamic engine's fuel efficiency is dependant on its expansion ratio during the work cycle. That's why a diesel engine is more efficient.

    And the expansion ratio is of course directly coupled to the compression ratio. In car engines, gas and diesel, expansion ratio is of course the same as compression ratio. But even if we had an engine where this is not necessarily true, like a gas turbine, the useful expansion ratio would still be closely coupled to compression ratio.

    And there you have it.
     
  7. bobsmith1492

    bobsmith1492 Diamond Member

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    QFT
     
  8. ratpie

    ratpie Junior Member

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    Actually...that is not 100% accurate. Theoretically an engine's compression ratio should be the same as the expansion ratio only because you are assuming the combustion to occur at the TDC (top dead center).

    However, in reality the combustion usually occurs something like 10 or 12 degrees (if i remember correctly...i might be wrong with the figures) after TDC. Thus, the expansion ratio will in fact become smaller than the compression ratio.

    here is a really interesting research done by Mazda on their latest Sky Activ Diesel engine. They've DECREASED the compression ratio to get an INCREASE in the expansion ratio! pretty sweet right?
    http://www.mazda.com/mazdaspirit/skyactiv/engine/skyactiv-d.html
     
  9. gsellis

    gsellis Diamond Member

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    RPM and compression are unrelated. The higher RPM is based on mass and inertia of the components. The higher the RPM, the more events per minute, equals more output. The higher compression, the more energy injected per charge, the more output.

    As far as RPM, it is inertia and mass that make for a redline (and ability to inject and scavange). Piston engines with spring driven valves get up to about 15k before they top out at the max range a spring can return a valve back to its seat. Big, heavy pistons cannot change direction past a point either (which is the Mustang vs the Hayabusa) without causing failures in the connection rods or crank connections. F1 cars use exotic alloy pistons and rods for weight and pneumatic valve actinuation (no springs) to get up in the 18k range. More RPMs, more horsepower.
     
  10. MB2012

    MB2012 Junior Member

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    Does anyone know how the engine compression ratio may affect engine reliabilty and longevity? For instance, is a 12:1 compression ratio engine more or less reliable than a 10:1 compresion ratio engine? Specifically I am looki at MB E series engines. Thanks.

    Sandy
     
  11. AD5MB

    AD5MB Member

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    compression ratios dropped when smog became an issue. that might be due to octane ratings dropping at the same time.

    high compression concentrates heat in the combustion chamber. high CR engines cook their rings early
     
  12. imagoon

    imagoon Diamond Member

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    Component for component a higher compression ratio increases stress on the moving part and the seals. In theory 12:1 will have less life than 10:1. Mostly because increased stress can wear the crank, bearings, rods, pistons and piston rings.

    Other side-effects such as increased blow by due to higher cylinder pressures pushing the fuel charge in to the crank case and past the valves. Value leaks lower net compression, blow by does the same but also contaminates the oil while can increase wear.

    Typically higher compression engines at least have stronger pistons and rings. So this is negated quite a bit. Most engines are designed to make it to nearly 200k miles (for normal production cars) so I wouldn't be worried about a 12:1 vs 10:1 in a production car.
     
  13. Lemon law

    Lemon law Lifer

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    Its my understanding that its even more complex than described when it comes to the emissions from gasoline engines. The very factors that cause the best efficiencies tend to skyrocket NOx production and on the other side one gets increased carbon monoxide and other nastiness.
     
  14. Yuriman

    Yuriman Diamond Member

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    This is partly why a diesel is more efficient. A diesel has slightly higher peak efficiency because of higher compression, but most of the real world efficiency gains are at part throttle. In a gasoline engine, half throttle steals efficiency by both robbing the engine of some power to create and maintain a vacuum, and effectively cuts the compression in half as well. Deisels do not generally have throttle plates and thus do not create vacuum, and always run at maximum compression, so the real world efficiency is vastly superior.
     
  15. NeoPTLD

    NeoPTLD Platinum Member

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    Gas engine is a point source ignition. Diesel is compression ignition, so fuel mist in hot air ignites at the same time. It provides for more complete ignition than having ignition at one point.

    The efficiency of car diesel is higher, but not to the point of direct relationship you see in "mpg" metric between a gas car and a diesel car.

    mpg is a good unit for comparing machinery using the same fuel, but its not a true measure. Diesel in part gets higher mpg, because energy contents per volume is higher with diesel.

    An objective efficiency comparison is comparing how much MJ of fuel energy content it takes to produce one MJ extracted at the shaft.
     
    #14 NeoPTLD, May 1, 2012
    Last edited: May 1, 2012
  16. imagoon

    imagoon Diamond Member

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    In a perfect fluid world yes. However nearly all diesel engine use a pointed tip on the piston to produce a hot point the ignites the diesel air mix. Detonation is just as damaging to a diesel as it is to a gas engine. They use the point to control the burn front. A large chunk of diesels efficiency does come from the lack of a throttle plate.
     
  17. NeoPTLD

    NeoPTLD Platinum Member

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    Can you provide some references so I can review the significance of this?
     
  18. NetWareHead

    NetWareHead THAT guy

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    The work put into cooling the incoming air (like when cars employ an intercooler) is so that air entering the cylinders is cold and denser in oxygen. Warm air is less dense and contains less oxygen.
     
  19. imagoon

    imagoon Diamond Member

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    I'll work on it, popular mechanic had a decent write up, I'll look for a web version of the article. While not directly what you wanted, you can find information about it using "part throttle plate pumping losses" as a search term. Since most [if not all gas] cars spend nearly all of their life at part throttle, creating vacuum they waste more rotational energy creating that vacuum. Diesel keeps the intake open to atmosphere and varies the fuel injected to power requested. This means there is less to pump against. Obviously the heads / intake paths will add to pumping losses but they should be comparable assuming the diesel and gas engines are of comparable size and design.

    A more "seat of your pants" way to see the affect is to use engine braking. Gasoline engines tend to have good engine braking while Diesel often needs to employ J-brakes to create any significant braking effects.

    Links as I find them:

    http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine
    BMW stating the new F10M5 Valvetronic system is 10-15% more efficient due to removing the throttle plate:
    http://www.bmw.ca/ca/en/insights/technology/efficient_dynamics/vehicles/technologies.html
     
    #18 imagoon, May 2, 2012
    Last edited: May 2, 2012
  20. Scout80

    Scout80 Member

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    After the air is brought into the engine and warmed up it will still contain the same number of oxygen molecules as it did in its cold state because the intake manifold is essentially a closed system. The oxygen can only go into the cylinders for combustion it can't escape the system. Even if a wastegate is used to relieve excess pressure the air molecules lost through the gate are quickly replaced by the turbo.
     
  21. imagoon

    imagoon Diamond Member

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    Warm air contains less Oxygen than cool assuming same volume and pressure. Intercoolers reduce the heat content of the air which increases the density of the charge. This will result in a higher oxygen content in the cylinder by weight assuming volume and pressure has remained constant. This also helps prevent detonation by reducing the charge temp.

    The wastegate itself is on the exhaust side where it diverts exhaust pressure away from the turbine to control the maximum pressure seen by the engine. It by itself has no effect on the charge otherwise.

    The gist of this means that 1 liter of air @ 100C will be lighter than 1 liter of air @ 0C. The 1 liter of 0C air thus can burn more fuel before the oxygen is fully consumed. Same idea applies in a forced induction motors even at 2 atms. 1 liter of 100C ~30psi air has less oxygen than 1 liter of 30psi 0C air.

    So yes the air warmed up contains the same number of molecules, how many in what amount that get in to the cylinder changes based on temp of the air charge.
     
    #20 imagoon, May 2, 2012
    Last edited: May 2, 2012
  22. engibeer

    engibeer Junior Member

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    Since you have already stated your inability to grasp the concept of Carnot theorem, let me make it a little simpler.

    Basically, a volume of gas is effected by one of two forms of energy

    1- heat

    2- pressure

    If you increase one or the other, you are raising the energy contained within the volume.

    You can add heat, which will result in either A: an increase in volume, or B: an increase in pressure, or C: both an increase in volume AND pressure

    There are two ways to increase the pressure of a volume. You can A: increase density (ie: filling an air tank), or you can B: decrease the volume (ie: "squeeze" the volume as a piston does)

    Any of the above 3 options (1 for heat input, 2 for increasing pressure), will have a positive effect on the energy contained within the volume.

    Heat is the most efficient, as it is directly related to the level of energy itself.

    Now comes the magic. The faster you raise the pressure, or the ratio, the more energy you are placing into the volume. Any time you add energy to a volume of gas, you increase molecular activity. When you raise molecular activity, those molecules will react far more violently after combustion has begun, releasing more heat. More heat equals more power in a combustion engine.

    One member mis-stated an irrelevance of compression to rpm. When applied to engines, he couldn't be more wrong. Compression is just one means of applying energy. The speed at which you compress the air is also relevant. This is the reason why diesel cycle engines can be designed to idle at much slower speeds than gasoline engines. Partly due to the reduction in pumping losses, as diesels are fuel throttled as opposed to induction throttled... the rest is due to the combination of two key operating characteristics:

    1- higher compression: because there is typically no induction throttle on a diesel engine (very few exceptions), the cylinder volume has more air (mass). Add to this the increased compression ratio of a diesel engine (we'll say 18:1 vs 9:1 of a gasoline engine), and you've just increased the amount of energy at TDC by about 150 times (15,000%). You may think this would be counterintuitive to place so much energy into the volume before combustion, but it's not. By increasing the energy in the volume, you have increased its volatility, and thereby its predisposition to combust with the introduction of the fuel.

    2- because so much more energy is placed into the volume in preparation for combustion via compression, the need for increasing the speed at which that volume must be compressed is much lower.

    There is a VERY close relationship between required compression ratio and speed (rpm). This relationship is known as "energy rise". If you compress the same volume to a given compression ratio at one speed (ie, 400rpm @ 18:1) in order to induce the same level of volatility at the point of ignition, you have to increase the speed if using a smaller combustion ratio (ie. 800rpm @ 9:1), or increase the mass (ie, supercharging or turbocharging).

    Increasing the compression ratio not only increases the volatility of combustion by means of decreased volume, but also forces more heat to be contained in the gas itself because you are lowering the surface area for that heat to escape. The higher heat results in higher pressure, which correlates to increased power.

    There are other limiting factors. In nascar engines, some teams sacrifice available compression ratio for increased combustion chamber volume. Even though the increase in combustion ratio would yield higher overall power throughout the rpm range, the greater chamber volume allows for increased volumetric efficiency at a particular desired point in the range (ie: 8,500-9,000rpm in the straights). This allows the engine to displace more than its actual displacement volume (ie: a 5 liter piston displacement engine running at 110% volumetric efficiency is actually displacing 5.5 liters on each revolution).

    This is why it is such a delicate balance to build a great engine. Everybody understands the four stroke cycle... I can teach it to a five year old. It's just your basic "suck, squeeze, bang, blow". Its all the other factors that make it difficult. True, increased compression typically yields increased power throughout the rpm range, but sometimes that's not what you need. Want to make it really difficult? Add in forced induction. Now that normally aspirated engine that was running at 9:1 compression, is suddenly pumped up to 18:1 effective compression if you have 14.7lbs of boosted induction at the valve seat.

    Please forgive the typos and run-on sentences, as this post was hastily typed on a mobile phone.
     
  23. engibeer

    engibeer Junior Member

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    Funny tidbit for you:

    Even though the the volatility of a combustible mixture is higher with an increased compression ratio, in spark ignited gasoline engines the spark requires MORE energy to ignite the mixture, due to the increased resistance in the spark gap (higher compression, higher density, higher resistence).

    By the way, whoever said that increasing compression ratio increases the atomization of the fuel was dead wrong. Unless you are shaping the induced swirl of the volume during compression (domed pistions, swirl angled port/valve agle, etc.) the droplets of fuel are actually more predisposed to condense, rather than evaporate.... hence the reason why the tank on an air compressor has a pitcock on the bottom... to drain the water that condenses in the tank after it's been pressurized.
     
  24. Burpo

    Burpo Diamond Member

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    One top fuel dragster 500 cubic inch Hemi engine makes more horsepower than the first 4 rows of stock cars at the Daytona 500.

    It takes just 15/100ths of a second for all 9,000+ horsepower of an NHRA Top Fuel dragster engine to reach the rear wheels.

    Under full throttle, a dragster engine consumes 1-1/2 gallons of nitro methane per second; a fully loaded 747 consumes jet fuel at the same rate with 25% less energy being produced.

    A stock Dodge Hemi V8 engine cannot produce enough power to drive the dragster's supercharger.

    With 3,000 CFM of air being rammed in by the supercharger on overdrive, the fuel mixture is compressed into a near-solid form before ignition.

    Cylinders run on the verge of hydraulic lock at full throttle.

    At the stoichiometric (stoichiometry: methodology and technology by which quantities of reactants and products in chemical reactions are determined) 1.7:1 air/fuel mixture of nitro methane, the flame front temperature measures 7,050 deg F.

    Nitro methane burns yellow... The spectacular white flame seen above the stacks at night is raw burning hydrogen, dissociated from atmospheric water vapor by the searing exhaust gases.

    Dual magnetos supply 44 amps to each spark plug. This is the output of an arc welder in each cylinder.

    Spark plug electrodes are totally consumed during a pass. After halfway, the engine is dieseling from compression, plus the glow of exhaust valves at 1,400 deg F. The engine can only be shut down by cutting the fuel flow.

    If spark momentarily fails early in the run, unburned nitro builds up in the affected cylinders and then explodes with sufficient force to blow cylinder heads off the block in pieces or split the block in half.

    In order to exceed 300 mph in 4. 5 seconds, dragsters must accelerate an average of over 4G's. In order to reach 200 mph (well before half-track), the launch acceleration approaches 8G's.

    Dragsters reach over 300 miles per hour before you have completed reading this sentence.

    Top fuel engines turn approximately 540 revolutions from light to light! Including the burnout, the engine must only survive 900 revolutions under load.

    The redline is actually quite high at 9,500 rpm.

    Assuming all the equipment is paid off, the crew worked for free, and for once NOTHING BLOWS UP, each run costs an estimate $1,000.00 per second.

    Putting all of this into perspective:

    You are driving the average $140,000 Lingenfelter 'twin-turbo' powered Corvette Z06. Over a mile up the road, a top fuel dragster is staged and ready to launch down a quarter mile strip as you pass. You have the advantage of a flying start. You run the 'Vette hard up through the gears and blast across the starting line and pass the dragster at an honest 200 mph. The 'tree' goes green for both of you at that moment.

    The dragster launches and starts after you. You keep your foot down hard, but you hear an incredibly brutal whine that sears your eardrums and within 3 seconds, the dragster catches and passes you. He beats you to the finish line, a quarter mile away from where you just passed him.

    Think about it, from a standing start, the dragster had spotted you 200 mph and not only caught, but nearly blasted you off the road when he passed you within a mere 1,320 foot long race course.

    ...... and that my friend, is ACCELERATION! Performance with no regard for economy! :)
     
  25. imagoon

    imagoon Diamond Member

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    2 year old necro post?
     
  26. Liberator21

    Liberator21 Golden Member

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    Keep the necro alive!

    All I know is I had a Cadillac CTS with a higher compression ratio that would run like s*** if I didn't put the expensive gas in it.

    I also had a Yamaha R1 with about a 15k redline and I strongly believe they aren't as reliable. Sure they are extremely reliable while operating in its 'lifetime' but its lifetime isn't that high. That's why you don't see a lot of motorcycles pass the 50k mile mark.