Limitations and Threshold of Combustion
The basic ingredients needed to achieve “Combustion” in any IC engine are, Oxygenated air along with a blend of Fuel which needs to be compressed appropriately within the cylinder and ignited. The next fastest thing ever happening inside a gasoline engine is a blue bolt of spark, flying across the electrodes at the speed of light to initiate the process of combustion within the cylinder. The subsequent “Bang” unleash hot expanding gases that punch the face of the piston down the cylinder to start the process of a twisting energy called “Torque” across the crankshaft with the help of a connecting rod.
The torque produced out of combustion directly relates to the subsequent pressure releases out of a rapid chemical and thermal reaction between the trapped hydrocarbons and the compressed air locked within the cylinder. These crucial moments of “expansion” within the burning gases generated out of combustion between the air and fuel reflect the “thermal efficiency” of any I C engine, may it be a F1 engine revving upto 18,000+ RPM or a ultra low revving big diesel propelling Ocean liners, all trying to make the best out of the “ Bang ” with the “ available air and fuel mixture locked within the cylinder.”
The irony of the story is, once “Combustion” is initiated within the combustion chamber, there is very little control over the progress or process of combustion till all the charge goes up in flame and smoke. The rate of burn within the charge during this process of combustion are determined by the in-cylinder pressures and temperature build-ups due to the increase in density within the charge prior to ignition, mixture formations within the compressed charge and the related Ignition timing’s decide the rest.
Ideally accepted ratios of such a charge are between 14 to 15 parts of air ( by weight ) along with “1” part of hydrocarbons in the form of liquids or gaseous fuels. Fuels come in a variety of combinations and are basically available in the form of “hydrocarbons”, i.e., carbon and hydrogen atoms bonded together and having a particular atomic numerical structure meant for a specific purpose to “fuel and fire” I C engines.
Fuels like kerosene have lower antiknock values, i.e., lower flash points and are best suited for low compression engines in comparison to petrol or aviation gas which can withstand much higher compression ratios. Ave gas - 100/140 have the highest tolerance to antiknock in the list of conventional combustible fuels used in I C engines. Diesel fuels on the other hand lack the volatile properties and do not evaporate like petrol, hence need very high conducive temperature surroundings to activate ignition and the desired rate of combustion. Therefore diesel engines invariably operate on much higher compression ratios in comparison to gasoline engines to achieve the desired rate of combustion. Normally 20 to 1 compression ratios or more are common in diesel engines as they build up the ideally required temperatures within the trapped air prior to diesel being timed and injected to induce self-ignition within the charge.
Diesel fuels due to their potency especially under higher temperatures and pressures unleash greater heat energy out of combustion thereby generating far higher pressures out of expansion within the cylinder than petrol engines can during the process of combustion.
Due to these basic inherent advantages, one being compressing the air to a much higher degree before ignition, followed by rapid combustion that gives diesel engines an advantage to produce higher torque outputs at lower RPMs than petrol engines can. In comparison petrol has a far lesser tolerance to heat build ups due to its volatile properties and inherent lower flash points in comparison to diesels. Petrol engines therefore need to run at lower compression ratios which need to keep the temperatures within the charge well below the flash point of the fuel in use before “spark ignition” triggers the actual process of combustion. Petrol engines must be spark ignited appropriately to achieve a rapid burn rate within the trapped volatile air and fuel charge to produces the desired uniformity of pressures that produce effective torque and power outputs out of combustion.
Hence the distinct differences between spark ignition and compression ignition engines are: Spark ignition engines (SI) need to use volatile fuels like kerosene, petrol or higher grades of premium petrol or alcohol blends such as methanol, etc. or other combustible gases that need to evaporate and mix with the incoming air at a very high rate to form into an ideal volatile charge and further rely on a source of ignition such as a spark to initiate the process of combustion at the right moment. Further more spark ignition engines need to operate at lower compression ratios, which in fact need to be lower than the flash point of the fuel in use to prevent erratic patterns of combustion. As erratic patterns of combustion could otherwise lead to a single collective explosion normally referred to as “Detonation” aided by “Pre-ignition” than the desired controlled rate of burn during combustion that produces useable torque and power with uniformly.
On the other hand, self-ignition or compression ignition engines triggered by direct or indirect fuel injection operate on fuels which are far less volatile and viscous needing conducive higher temperature build ups within the compressed air to support self ignition. This is achieved by injecting diesel at very high pressures in the form of a fine spray into the combustion chamber nearing TDC (Top Dead Center) when the temperature and pressures within the compressed air are in an ideal state to cause friction between the two like striking a match stick. This form of friction between the two unleashes the trapped thermal energy within the charge during the rapid process of combustion in compression ignition engines. Designers have tried to introduce pre-heated diesel or less combustible fuels along with the incoming intake charge into the cylinder and have attempted to ignite this with the help of spark but have failed to achieve proper ignition despite the applied compression that generates the necessary heat in the charge. This is mainly due to their poor evaporating and mixing qualities that are prevalent in less volatile viscous fuels like diesels and vegetable oils with the in coming air. “Gases can be compressed into liquids but liquids cannot be compressed any further”!
Hence diesel engines are built Robust to withstand all the over workouts needed to sustain the higher degrees of vibrations due to the “harsh and noisy” combustion while petrol engines need to operate well below the red line of detonation by making sure the blue bolt of lightening appears at the right moment to initiate the process of Combustion ! Moreover diesels spend a portion of the “earned torque” to compress the air within the cylinder to achieve the much needed higher in-cylinder temperatures and pressures that are essential to initiate a proper ignition process followed by combustion. Despite all this, diesel engines still manage to produce better thermal efficiencies out of less volatile fuels than petrol engine do with volatile fuels !
Hopefully having understood these basic differences between the two; Let us observe combustion in slow motion and see how heat buildups and mixture formations within the charge prior to ignition support efficient patterns of combustion that produce torque with the least interference from pre-ignition or detonation next, in Part II
To understand the effects of compression followed by ignition and the actual time available for expansion during the combustion cycle occurring within the cylinder – A better understanding of piston oscillations within the cylinder in relation to crankshaft movements needs to be clearly understood.
To start with, the piston “reverses” its directions within the cylinder at the end of every stroke i.e., once at TDC (Top Dead Center) and once at BDC (Bottom Dead Center). The crankshaft determines the “throw” i.e., piston travel from TDC to BDC, and it is often referred to as the “stroke length” of a particular engine. The length of the connection rod in relation to the stroke length determines “piston acceleration and deceleration” speeds in relation to RPM and the time frame the piston spends to reverse its direction at both ends of the stroke. The capacity and the displacement of the engine is finally decided by the piston diameter best suited for a specific purpose. The age old rule of thumb suggests that a long stroke produces torque while a short stroke produces power ! Lets get a better understanding of this statement and its background.
Looking at these fixed parameters of the up and down movements of the piston gives us an indication of how the piston accelerates and decelerates differently in each quarter of the 90 degrees in relation to a full circle rotation of 360 degrees of the crankshaft based on the stroke length in combination with the length of the connecting rod. These layouts eventually convert the up and down movements of the piston into rotary motion at the end of the crankshaft with the help of a connecting rod.
“Taking a closer look” - During the first quarter or the first 90 degrees movements of the crankshaft starting from TDC, the connecting rod moves the piston down a distance more than half the stroke length within the cylinder thereby giving the ideal advantages to translate the generated thrust out of combustion in the form of expansion to pedal the crankshaft into rotation. The next quarter of 90 degrees movements of the crankshaft towards BDC the rate of descend of the piston within the cylinder is relatively slower as it reaches the end of the stroke. The piston further spends a lot longer time in the vicinity of BDC due to the pendulum effects generated out of the length of the connecting rod in relation to the gudgeon pin at BDC with references to crankshaft movements. The rate of acceleration in the piston after reversing its directions from BDC is slower in comparison to the last quarter that thrusts the piston upwards towards TDC on the compressing stroke completing a full circle of rotation.
One can now see that the piston is at a much higher rate of acceleration and deceleration towards TDC in both the quarters i.e., during the compression stroke and the power stroke utilizing a larger portion of the cylinder volume to its advantages in relation to crankshaft movements. The piston further spends far lesser time in the vicinity of TDC than at BDC because of the connecting rod length and layout in relation to the stroke length further determined by the crankshaft rotation. These crucial patterns of piston movements in relation to their oscillation have a direct bearing and are in relation to the length of the connecting rod with reference to the bore and stroke of any reciprocating I C engines. These dimensions, to a great extent, spell engine characteristics. “F1” engines are built with a specific purpose to derive maximum power out of the lightest and most compact three-litre V10-cylinder engines ever built. This is achieved by an over square layout i.e., 80mm x 50mm stroke with the shortest possible connecting rod needed to compact engine dimensions and to withstand the very high piston accelerations and decelerations experienced at 18,000 + rpm’s. The 80+ mm bore diameters make room for the placement of at least three big inlet valves and two exhaust valves to efficiently breath and produce maximum Torque and Power in the vicinity of 16,000 + revs and withstand the constant thrashing needed to last a full race and WIN !
To further understand the limitations of combustion, piston movements in relationship to crankshaft movements and their effects with RPM are very essential. The harmonics and wave patterns generated out of piston acceleration and deceleration in relation to the time frame the piston spends in the vicinity of TDC go hand in hand to understand cylinder filling pulses which directly relate to in-cylinder pressures generated out of combustion which finally act upon the piston to transfer and translate this developed thrust ideally through the connecting rod to spin the crankshaft and store the developed torque in the form of energy at the end of the shaft in a flywheel.
Different types of fuels have different burn rates. Fuels of any type need precise “ignition timing” to initiate combustion, as too early will get the hot gases to expand at their recommended rates while piston movements are very slow followed by a momentary halt at TDC thus generating excessive pressure build ups within the combustion chamber bringing the remaining unburned charge close to their flash points resulting in an instant explosion, sending unbearable shock waves, i.e., Collision of two flame fronts one “burning” and the other “exploding” ! The consequences of this phenomenon are even worse under loads as the air fuel mixtures would be richer and more potent. Such explosions in continuity would spell disaster to any engine and are best known to cave piston and disfigure the internals of the combustion chamber followed by blown head gaskets due to the excessive heat and pressure build ups further accompanied by shock waves which occur beyond the speed of Mach – 1 followed by sonic booms heard in the form of pinging or knocking !
“Hence ignition timing and the flash points of any particular fuel go “hand in glove” with the antiknock value of the fuel in use to achieve the best out of in-cylinder combustion under compression in any I C engine”.
This makes the subject even more interesting ? Let us take a closer look into indoor combustion taking place inside the cylinder of any I C engines. Assuming we had an ideal charge consisting of the right grade of fuel along with the right proportions of air, entering the cylinder, through the just opening inlet valve(s), flowing past ideal inlet port(s) during the inlet phase due to the low pressures generated out of a rapidly descending piston within the expanding cylinder, during the first quarter past TDC would result in - The piston would creating a volume more than half the volume of the cylinder in the first 90 degrees of crank movements due to the connecting rod lay out – The inlet charge would now be flowing rapidly into the cylinder diving straight towards the descending piston crown due to the surroundings of a partial or full vacuum existing above the fast moving piston crown within the cylinder ( The degree of low pressures would depend on engine rpm’s and other factors related to engine lay outs based on the physical dimensions) At this stage the valve(s) would be wide open nearing max flow achieving maximum flow velocity in the incoming charge flowing into the cylinder – In the next quarter, the piston would be gently slowing down while the intake pressures in the fast moving charge flowing through the ports would take over due to the inertia built ups in the inflowing charge which further continue to flow towards the piston crown due to the low pressures still existing within the expanding cylinder – Nearing the end of the stroke the piston would be slowing down gently to reverse its direction at BDC due to the cushioning pendulum effects created out of the connecting rod layouts - The third quarter, would result in optimum cylinder filling due to the slower upward movements of the piston gathering speed while valve closing needs to be precisely timed to ensure maximum cylinder filling and further preventing the least spit back. (This is where variable valve timing ( VVT ) setups gain tremendously over conventional setups to achieve optimum cylinder filling and offer a much wider power band with the least spit backs ! Closing of the inlet valve(s) commences the actual “compression stroke”.
At this stage compression actually starts to form and build up above the piston crown rather than the cylinder head due to its rapid upward movements followed by the larger portion of the compressing charge trapped within the cylinder are still moving towards the fast ascending piston due to the mass inertia generated in the incoming charge during the initial inlet phase aided by gravity – In the last quarter, the cylinder is sealed while piston speeds start to pick up to its maximum speeds due to the angular velocity generated out of crank movements at mid stroke.
At this point the majority of the mixture trapped within the cylinder starts to get compressed above the piston crown in layers (due to its weight) these effects could exceed by several 100 Gs if not more depending on engine RPM’s. What follows is accompanied by heat build ups within the charge due to the rapid increases in density leading to separation in the air and fuel mixtures due to their individual properties ?? – This occurs despite all the induced turbulence in the incoming charge developed out of tumble or swirl motions. As the piston nears TDC, cylinder volumes start to diminish to the least determined by the compression ratio ultimately forming the combustion chamber at TDC ( while ignition has just been initiated before TDC ? ) followed by the larger portion of the stagnating compressed charge above the piston crown being forcefully “tossed” upwards towards the cylinder head due to the piston rapidly reversing its direction at TDC – This effect of “ Throw ups “ further aided by appropriate squish layouts in combustion chambers accelerates the process of combustion in I C engines – The start of the next quarter past TDC, ignited flame fronts located in the combustion chamber rapidly spike outwardly towards the piston crown and consume the majority of the charge “thrown upwards” towards the advancing flame fronts building up very high pressures generated out of the rapid combustion process taking place within the confined space which eventually result in punching the piston down the cylinder to appropriately start a power stroke that produces optimum torque out of this “perceived combustion”. In all probability the process of combustion is completed by the time the piston descends 1/10 the stroke length from TDC followed by rapid expansion in the hot gases which effectively punch the piston crown to achieve an efficient power stroke out of combustion during this quarter of crank movements utilizing a larger part of the cylinder to its advantages “suggesting” – The age old, rule of the thumb, “A long stroke produces torque while a short stroke produces power” ?
Lean mixtures generate lower pressures out of combustion due to a slower ignition phase followed by slower flame front propagation with in the charge and burn much longer in the cylinder decipating most of the generated heat into the walls of the combustion chamber and cylinder than punching the piston to derive an effective power stroke. On the contrary, richer mixtures burn much faster but run out of oxygenated air to burn complete and produce very less expansion due to the lack of heat in the burned gases producing an ineffective punch. Lean mixtures heat up engines while rich mixtures cool engines and fail to produce power ! “The ideal mixture formations generate optimum torque” – Provided we get the ignition timings right by making a compatible compromise between compression pressures and heat buildups within the charge prior to ignition. Ideal combinations deliver the most effective punch which are bound to have a direct bearing on sustained torque each cycle thereafter reliably and efficiently over a broad band for long periods of time out of I C engines.
Briefly these in-built inherent features of the “Century old” perfected IC engines bring us to the “Limitations and Thresholds of In-Cylinder Combustion” !
The other facts that emerge out of the “4 stroke” layouts are that the major portions of the intake charge invariably rushes into the expanding cylinder and start to pile up and compress above the piston crown while the cylinder head which houses the source of ignition relies on the piston to sling the majority of the charge towards the cylinder head at the end of the stroke to effectively start the process of combustion while “two stroke” engines score an advantage over four strokes as the transfer ports in two strokes are ideally aimed towards the cylinder head and are further aided by the fast ascending piston to initiates the process of combustion more effectively, thus requiring lesser ignition advances than four stroke SI engines !? Diesel engines inject the spray of Diesel directly towards the fast ascending piston crown which houses the larger part of the compressed air and score over the rest by achieving better thermal efficiencies out of combustion.
Finally coming down to “Thermal Efficiencies”? Verses “Volumetric efficiencies” ?? – The first one “spells” efficiencies of fuel burning within the trapped charge that produce heat which effectively punch the piston down the cylinder while “Volumetric efficiencies” reflect cylinder “filling” that produce power with respect to RPM ! We all know engines “PURR” at idle speeds and while cruising but most definitely start to “ROAR” when demands for power are made. The day I C engines can only “Purr” and yet deliver big surges of power on demand without the accompanied “Roaring” ! “Will be the Day” ! We would have certainly improved the “thermal efficiencies” out of present day I C engines and would have pushed their efficiencies well beyond the present thresholds of 25 to 32.5% as achieved of today ! ?
We hope all this happens sooner than later considering the extensive efforts made by World Wide R and Ds spending Millions if not Billion of $$$$s on cutting edge technologies “All in the hope of trying very hard to “ignite the charge more effectively” than before to achieve this illusive “Bang” that produces more “torque” than ever out of the least inputs in fuels followed by the least emissions out of I C Engine Combustion. ?
“We cannot ignore the fact” ? – That we just can’t beat the simplicity of the existing century old perfected I C ENGINE - As they can kick start to life in the first rotation and comfortably “idle”with consistency. They can deliver sufficient power economically at the asking too ! With reasonable inputs of fuel. I C Engines of today can reliably deliver sustained power economically over long periods of usage too, requiring very little maintenance. When required they can be switched off with ease and stored for long periods requiring no further “recharges” for some time to come.
“All that they definitely need to do is ? - Just get a little more “Efficient and Tidier” with a lot lesser emissions and CO2 in time to come” !
Who is around to - “ Bell ” the “ CAT ” ?