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Some interesting answers have been posted. But the question is poorly defined for several reasons. The snap impression most readers would assume is the word ,Hemi ,is exclusive to the Chrysler Corporation with the introduction of the 331 cubic inch Big Block Firepower Hemi-,spherical ,engine in 1951. This was the beginning of a long stretch of Mopar big blocks designed with a crossflow Hemispherical cylinder head culminating with the world renown 426 CID V-8 Hemi installed in Chrysler, Dodge and Plymouth cars known as muscle cars between 1965 and 1972. The Hemispherical pistons and corresponding cylinder head design was not new. In fact, this combination was first implemented by Italian car builder Pipe in 1901. Fiat soon followed in 1907 with its Grand Prix race car engine. By 1915, Alfa Romeo, Mercedes Benz, Peugeot, BMW soon followed suit. The first 4 valve hemi head was built by Stutz of Indianapolis, Indiana in 1912, 39 years before Chrysler introduced its very crude design. Every automobile manufacturer that produced a minimum of 500 engines, has designed and manufactured a hemi head and piston engine. And I do mean, ,every big player. Hemispherical designs offer the following performance advantages; High airflow angle intake and exhaust ports Resistant to reversion High mechanical compression ratio Short connecting rod, high rpm horsepower (3,500 - 7,000) Long connecting rod, low (3,500 - 5,500) rpm torque Centered or offset spark plug(s) position for even fuel burn 180 degree crossflow Equalized intake pulse (90 degree even fire) timing for V-8 and V-16 models Modern Top Fuel Drag race engine (500 cubic inches) with 12V71 supercharger is capable of up to 8,500 - 10,000 HP for 4 seconds @ 8,400 rpm The design also has drawbacks; Restricted exhaust flow because of trapped exhaust gas on intake side of piston dome. Inefficient in low compression engines (less than 10.5:1). Requires high octane fuel to take advantage of high compression design. Requires aggressive camshaft duration and lift in high performance application Poor fuel burn design if true Hemispherical design is applied Is the Hemi the best design? Nowhere close to the best. By the 1980’s, the design radically morphed and only used in high performance sports cars. Most Japanese 4 cylinder high performance applications reduced the Penta piston top to have a very shallow slope to eventually eliminating the above block deck protrusion and focusing on the cylinder head chamber shape instead. This is the same route Ferrari decided upon, beginning with its famous 308 Quattro valve cylinder head. It used a perfectly flat piston top and matched it with a closed chamber inverted Penta roof design. Almost all modern 4 valve cylinder heads use this approach. See Nissan VQ picture shown below. This cylinder head chamber design has been around since 1931, beginning with the Rolls Royce Merlin PV-12. Capable of producing between 1,000 and 2,150 HP at a mere 3,000 rpm, the 27 liter (1,654 cubic inches) was the most powerful 12 cylinder engine in its class between 1936 and 1948. This engine was used in the Hawker Hurricane, Supermarine Spitfire, North American P-51 Mustang, De Havilland Mosquito and Avro Lancaster. Photo © Nissan 4 valve (VQ) cylinder head. Shown in the picture is the most common 4 valve cylinder head shape used worldwide. Almost equal size intake and exhaust valves. The intake and exhaust ports are raised to improve airflow. Note the shape of the closed chamber that forces compression (air fuel charge) to the center of the cylinder head where the spark plug is positioned. Also shown is the protruding wedge (top middle of the chamber) between the exhaust valves to promote equalized exhaust flow towards each exhaust valve. In conclusion, the true Hemispherical piston dome and cylinder head chamber is inferior to a flat top piston with penta-roof closed chamber cylinder combination. Photo © Diamond Pistons 2018. Classic Chrysler 426 piston. Note the massive inverted V dome. This racing piston has oil ring gas ports shown in the lower ring groove. The piston dome itself is a close copy of the design Chrysler used between 1959 and 1971. Here's the real dope on Chrysler's current (2003–2019) ,Hemi ,design. It is not even close to the original design Chrysler used in 1969 or 1970. The Demon 6.4 L 525 HP engine is nowhere near a true Hemispherical design. Photo © Hot Rod Magazine 2019. Chrysler 2018 6.4 Hemi cylinder head. FCA decided to use a semi-open chamber design with only two valves with unique twin spark plugs. By using 2 spark plugs, with open chamber architecture, engineers maximized the use of variable camshaft timing. This improves fuel burn in the low and mid-range rpm powerband. The engine uses a flat top piston design, just like the Rolls Royce Merlin and Nissan VQ. The Demon Gen 3 FCA Hemi is a superb engine, it’s just not a real hemi. Only in name (marketing 101) and trademark. Happy Motoring!
Funny thing about spark plugs is that they do not fire by themselves. A great many things are going on before that spark happens, completely out of the control of the spark plug. The ignition switch can be faulty, battery flat, coil can be faulty, exciter circuit and transistors can be faulty, high tension leads, distributor cap problems etc. Then if these are all OK, the spark plug could be cracked, have an eroded anode, carbon fouled or combustion deposits have built up and shorted out the anode to cathode clearance. Eventually you find out that it is easier and cheaper to simply replace all the spark plugs with modern Iridium type, and not have to worry about them again for 100,000 kms.
I would actually recommend using electronic ignition, rather than anything mechanical, because it’s going to be more transparent, easier to construct, easier to maintain, and easier to modify. I’ve found, as I get further into engine electronics, that most people scare themselves away from something that’s potentially fairly straightforward. I’m going to try to walk you through a very basic spark system design, which will hopefully form a solid base for you to work off of. From an electronics perspective, a spark is just another type of signal (like TV or radio), and an ignition system is a fancy way to make spark signals using the rotation of the crankshaft. An electronic ignition system has a three main components to it: A Rotary Encoder, (Generates a crankshaft signal) Spark Logic Circuitry, (Turns the crankshaft signal into a spark signal) Ignition Coil/Drivers ,(Amplifies the spark signal to fire the plug) I’m going to start with the simplest case, which is 1 piston, 2 strokes. For a single piston, 2-stroke engine, electronic ignition is ridiculously simple. This is because you don’t need to worry about the relative position of your crankshaft; you only have one spark to trigger, and you don’t need to worry if you’ve accidentally mixed things up. This means that part #2 of the system, your circuitry, is actually very easy. So easy, in fact, that you can pretty much almost cover it with your design of #1. First, you’ll need an encoder to make some sort of signal. Rotary encoders are pretty easy to work with, but we can get even more basic than that: Hall effect sensor [US5881LUA] ID: 158 This hall effect sensor drops its output voltage to 0 whenever the face that’s labeled is close to the north pole of a magnet. It also triggers very sharply, so it’ll be great for driving an ignition coil. There’s very little chance of us accidentally creating a fake signal from some stray EMF somewhere. You need to get some sort of magnet spinning on your crankshaft to create the signal; what I would recommend is getting a plastic gear that fits your output shaft which can house the magnet. You can cut out a bit of the edge of the wheel and glue in ,a wedge-shaped magnet, to create what is called a trigger wheel: You’ll have to fabricate some sort of mount so that the hall effect sensor stays really close to the edge of the wheel, even as the motor vibrates. Luckily, the sensor doesn’t really weigh anything. Normally trigger wheels in cars are labeled as [High Pulses - Skipped Pulses] to indicate what kind of signal is being generated. Something like 60–2 is pretty common, which indicates that there are 60 teeth on the wheel which generate a pulse and 2 teeth missing that don’t generate a pulse (to determine orientation on startup). We would call our simplified trigger wheel something like a 1–30 wheel. Next, we need to take this crankshaft signal and turn it into a spark signal. Our hall effect sensor is going to give us a signal that looks like this: Where every time the magnet passes, the voltage drops to zero. To trigger our spark plug, we actually need the opposite to happen: So we need a circuit that is capable of inverting that voltage. Since engines have lots of spinning metal in them (which can create EMF issues), something that can handle alternating current (AC) without frying is a good idea. This line driver is capable of doing that for us: ,SN7407N Texas Instruments | Mouser Finally, we have to turn our spark signal into an actual pulse of the spark plug. Luckily, some cars are designed with engine computers that only output a logic-level spark signal. The, GM LS2 engine, that’s used in various cars and trucks has a standalone coil that’s capable of taking our spark signal and firing a plug off of it: The reason I would use this particular coil is that it’s designed to be remotely mounted, with short little plug wires that stretch from the coil to your spark plug. This means that you don’t have to mount it anywhere precise, which makes your life a lot easier. These are sold online for pretty cheap, but only in sets of 8. If you need individual ones, the junkyard is your best bet. Unfortunately, the GM coils are a bit weird and want a 12V power source and a 5v signal. To deal with this, we can hook everything up to a car battery and put the signal circuit on a 5v voltage regulator: ,L7805CV STMicroelectronics | Mouser Now that I’ve given an overview of our components, here’s a diagram of how to set up the system: I’ll try to come back and make this diagram a bit clearer, but you can definitely use it to wire things up. All of the wires going into things are in the correct order, except that line driver. On that part, I’ve put the correct pins in red. Once the circuit is up and running, the trigger wheel needs to be put at the right angle, which in this case is basically “timing” the engine. You’ll need to figure out a what point the piston is at the top of its stroke, and then you’ll probably want to center the magnet on the Hall sensor, or maybe rotate it so the magnet is a little ahead of the piston, which will give timing advance. Timing advance is important because the explosion takes a while to happen, and so you get the most power out of your gasoline if you ignite it a little before the cylinder starts to decompress. If you want the timing to remain consistent after you set it, try to find a trigger wheel that has a set screw in it; once the timing is correct you can tighten it in place. To be absolutely clear, this is a very basic electronic setup and many improvements can be made. Besides a two-stroke engine, this can also run a 4-stroke single-piston engine, it’ll just fire off an extra spark during the exhaust stage (known as wasted spark). That’s usually not a problem, although if your timing is very off and your spark is really strong you might ignite the gasoline that’s entering the combustion chamber. If you want to add more pistons, you can simply create more of these circuits in parallel and stack a bunch of wheels and sensors on your output shaft. Different engines couple the pistons differently, so to do wasted spark you’ll need one trigger wheel for each set of pistons that are at the same angle on the crankshaft. For example, an I4 will need 2 triggers, while a V4 will need a full 4 triggers. The other option, if you want to get complex, is to replace that line driver with a microcontroller, which you can program to do whatever you want. If you have a single wheel with a lot of triggers, then your microcontroller can keep track of the position of the crankshaft and fire each spark plug individually. This is how a standard engine computer operates. Most engines will modify timing as they rotate more quickly, because you need more advance to make sure there’s enough time between the ignition and the decompression stroke. This circuit doesn’t have any provisioning for that, but if you add a microcontroller then you can program it to calculate RPM and adjust your timing however you want.
What is the reason pilots leave the brakes on when first raving the engines before takeoff? There are potentially three times (that I can think of offhand) that aircraft “rev” their engines before taking off. The first is associated with piston driven (general aviation) aircraft and the other two could be with any aircraft. Before a piston driven aircraft taxis onto the runway for take-off the pilot will perform what’s called a “Run Up.” This is when the pilot increases the power and checks the engine(s) to make certain that all cylinders are performing correctly, that the Magnetos are firing both spark plugs in each cylinder, that the oil temperature is hot enough for departure, that the fuel pressure is sufficient, etc.. Once the aircraft taxis onto the runway for take-off some aircraft (like mine) recommend that the pilot once again advance the engine’s power to an intermediate setting below full power just long enough for a final examination of all engine parameters before brake release and applying full power. If an aircraft has to make a “short field take-off” — presuming a shorter than ideal runway length — the pilot will usually position the aircraft on the runway and apply full power BEFORE brake release to maximize the aircraft’s acceleration in order to shorten the runway required before liftoff. In all of these instances, the brakes are to keep the aircraft from moving before the pilot is ready.
On most cars I’ve driven, a flashing check engine light means something is very seriously wrong. That means don’t drive the car till you’ve fixed it. An OBD2 scanner is pretty much a necessity for working on modern cars. They don’t even cost that much anymore. Plug it into your OBD port, and it’ll most likely tell you what’s happening to make the check engine light flash. Look up the code and error message on google or youtube, and you can most likely find a description of how to fix it. If it says something vague, and then think about what you did and what the problems might be. If changing the plugs was ALL you did, there are a few things that come to mind as the first things I’d check. Think about what you touched, and what those parts do for the engine. The plugs, wires and coil are all things you might have disturbed or miss-installed, and they all have to do with ignition. My thought is that you might have one or more misfiring cylinders. An engine will continue to run, although poorly, with a surprisingly large fraction of its cylinders failing to fire, but the computer that controls it will see this as a pretty serious problem, and either detect the misfires directly, detect the extra oxygen in the exhaust, or both. Are all your spark plug wires back in place and making good contact with the terminals, both at the plug end and the coil end? If you have ‘coil over’ plugs, are the connectors to the individual coils tight? Did you check gap on all your plugs before you put them in? If the gap is too small or too large, the plug can fail to spark or spark so weakly it doesn’t ignite the gas. Did you tighten all your plugs to the proper torque? A loose plug can result in a misfire due to lack of compression. Did all the plugs have their crush washers, or was one missing? Did you reinstall all the plug wires to the correct plugs, or are some of your cylinders firing out of sequence? Are all your spark plug wires in good condition, or were some damaged during your work? It doesn’t take much damage to high voltage wiring before sparks come out at the wrong places. Is the coil in good condition? Is it dry? If you had the hood open in the rain, a wet coil can malfunction, as rainwater is quite conductive at high voltage. The plastic shell on an old coil can get brittle, and working with it can crack the insulation. Oily gunk on the coil can bake into a carbon layer that’s conductive and short out the coil too.
As in most problems of this sort it's reasonable to take the metric to each logical extreme and examine the behavior to gain a good foothold on the issue at hand. If your spark timing is too retarded, the fuel will either ignite from the pressure alone (called 'dieseling') or, more likely, will combust so late that the expansion of the burning fuel mixture will be slower than the expansion of the combustion chamber volume as the piston drops and the fuel will do no noticeable work on the engine. This will severely limit your power production. What if you advance your spark timing too far (the other extreme)? Well, the burn will occur before the cylinder has reached TDC and all the energy being produced by the combustion will go to waste since it will be counteracting the rotation of the engine. Now if your fuel mixture burned immediately, you would have your spark timing set at TDC (top dead center) or just before. But you have flame front propagation throughout the cylinder that takes time so you ignite the fuel well before TDC so that peak cylinder pressure occurs at the most efficient time (more on that later). Now we have the upper and lower limit of spark advance determined. To find a more optimum location we have to look more closely at the burning of the fuel mixture. You have a few measurements to look at: SOC (start of combustion), COC (centroid of combustion, when half of the fuel mass has been burned), and total duration of combustion. This is usually measured in crank degrees. I'm going to assume that by 'delay period' you mean the difference between the spark plug firing and SOC, when the flame front actually starts to travel across the cylinder. Basically, there are multiple delays to look at. The delay between spark and combustion is the one you alluded to but you also have to wait as the flame front travels through the fuel mixture and burns progressively greater fractions of the mass (I use this phrasing because 'mass fraction burned' is common terminology in ICE research). The peak pressure of the burn is usually close to the centroid so the second delay is between SOC and peak pressure. This image will serve as a good visual aid for my explanation: The motored pressure trace is the mechanical compression that occurs from the piston rising and falling. It's obtained by motoring the engine with no spark and recording the pressure trace output. The MFB curve will yield the three characteristics I alluded to: where it becomes nonzero is SOC, where it equals 0.5 is the centroid, and where it becomes 1 gives the duration of combustion. You can see that the peak of the firing pressure trace is approximately over the centroid of combustion, though MFB profiles aren't always symmetrical so you might have a faster burn in the first half of the mass or some such thing. In short, setting your burn location is a balance of trying to place as much of the firing pressure trace over the downward stroke of the engine as possible. There are a number of other factors to consider, such as irregular flame front speeds and the like.
Do driving fast and turning off the car in the middle of driving affect the car? Here’s a short course in engine operation. When the engine is running the fuel pump provides fuel to the fuel injectors (or carburetor) which introduces fuel into the cylinder at the proper time during the engines’s Intake cycle. Given that the spark plug fires at the proper time —just before the piston reaches top dead center (TDC) on the Compression Cycle — everything works as it should. The Combustion Cycle is next and that is what creates the engine’s power. Finally, the Exhaust Cycle empties the cylinder of all remained gases. If the driver turns the ignition off but doesn’t put the car’s transmission in neutral, the engine, instead of driving the car is actually being driven through the transmission BY the cars momentum. What that means is that the mechanical fuel pump continues to introduce the fuel/air mixture to the cylinder. Here’s the bad part. The fuel/air mixture doesn’t get ignited so the Exhaust Cycle sends a rich mixture of fuel out the exhaust system. At this point, all it takes is a hot enough exhaust manifold or the ignition to be turned on again and there a reasonable chance you’lle get what used to be referred to as a BACKFIRE. Backfire’s are explosive and can potentially do damage to the car. So my recommendation is that drivers don’t turn the engine off when the car is running without at least putting the transmission in neutral.
No. Why? The engines and supporting components are constructed differently BECAUSE the fuels are so different in their properties. Diesel is ignited by compressing the air to a very small volume. This heats the air up. The diesel is then sprayed and combustion starts. There is no spark plug in a diesel engine. Some engines have ,glow plugs, but they are used to warm the engine and have a completely different function. Now petrol is ignited by a spark (that's what a ,spark plug, does). In a petrol engine, the air and petrol are mixed beforehand and are then injected into the combustion chamber and compression follows. The spark plug is then triggered to initiate combustion. In a diesel engine only the air is sucked in and compressed. At this high pressure, diesel is sprayed in. Now if you put diesel in a petrol engine, the mixing will not be good (because diesel is not as volatile and will not form droplets and vapors as easily as petrol, in fact diesel is fairly viscous) and the spark plug cannot initiate combustion. Take two rags. Dip one in petrol and the other in diesel. Try to light each one with a match. The rag dipped in petrol would catch fire instantaneously because of the petrol vapor surrounding the rag (and also because petrol is extremely flammable). You would have a harder time lighting the rag dipped in diesel with a match. And that's the reason why you can't put diesel in a petrol car. It just wont burn. Now about petrol in a diesel car, I can't think of a reason why it won't work for a short time but I suspect you would have detonation problems because of the high compression ratio. Combustion would start before top dead center and the force acting on the piston wont be synchronous with its position. I have to read up a bit on this issue. I hope someone corrects my answer. I don't think I have covered all the points. I will be back to correct my answer later. Engineering explained did a video in 2016: I would urge you to read a bit about automotive engines because the answer would be evident if you knew exactly what happens in a diesel/petrol engine. This is a good book, easily available online. After reading the other answers and comments, it is necessary to edit my answer as promised. A logical follow up question for this question is: ,Which is worse? Adding diesel to a petrol engine or vice versa?, The answer is the latter. The consensus is that diesel being considerably viscous, is used as a lubricant too. Addition of petrol to a diesel engine will spoil the lubrication system (pump, seals and other rubber linings) as said by ,Jeremy Miles,. If diesel is added to a petrol engine, you just need to drain the fuel lines and get rid of the diesel. The original question is very important as the answer to that question also kinda answers other questions like: Why is a diesel engine heavier than a petrol engine? Why is a diesel car more expensive than a petrol car? Why does a petrol car give better performance than a diesel car? Why is a diesel engine more efficient than a petrol engine? Why are most SUVs and big cargo vehicles powered by diesel? I found this page from the Castrol website which is very informative. Please go through it. Diesel vs petrol I won't comment on the above questions in this answer as it will dilute the content and start diverging from the original question. But please feel free to put up a question in the comments section.
It will run rough or not at all. I used to have a mobile tuneup business back when we still used points inside the distributor. I had a call from a lady whose car was running quite badly. I found nothing wrong and finally gave up. I called her later to find out what I had missed. It turns out a coworker had switched plug wires. Troubleshooting Firing Order A critical component of a functioning engine is the firing order of the cylinders. When the firing order is wrong or delayed, the engine does not run properly. The spark may be delivered to the cylinder when there is no fuel/air mixture or before it is properly compressed. One symptom of an incorrect firing order is the engine does not run. Turning the ignition does not start the engine. Causes of Wrong Firing Orders Every engine manufacturer provides a service manual detailing the correct firing order of their cylinders. Firing order begins with a spark plug wire leading from the number one cylinder to the number one position on the the distributor and the remaining spark plug cables connected to their respective cylinders. When one or more spark plug wires are crossed, the engine may not run, or runs roughly as the timing of fuel/air compression does not align with the delivery of the spark. Correcting Firing Order Locate the number one terminal on the distributor and ensure that its spark plug wire runs to the number one cylinder. Ensure that the number two spark plug wire goes to the number two cylinder and so on. The vehicle's service manual will indicate where the number one terminal on the distributor is and which cylinder is number one. Check for shorts, cracks and frays in the wires themselves to ensure that a short is not causing the wrong firing order. ,Symptoms of a Wrong Firing Order
Short answer: ,it probably won't have any effect, but there are reasons not to do it anyway. The "octane rating" of a fuel is basically a measure of how much a mixture made of the fuel and air can be compressed before it spontaneously ignites. During the compression cycle the fuel-air mix is compressed in the cylinder by the piston, which then is pushed back by the eventual explosion of the mix, creating power. In a gasoline engine, you ,don't want ,uncontrolled, spontaneous ignition (also called detonation) to occur; rather you want the fuel-air mix to ignite predictably and at the perfect moment, defined when the spark plug fires. If the octane of your fuel is too low for the engine, you may hear the the characteristic knocking or pinging sound that indicates detonation is occurring, signifying poor performance and probable damage to the engine over the long term. (I'm assuming we are talking about a modern [fuel-injected electronically-controlled ignition] engine here. If you have a carbureted engine and you don't know about engine timing and tuning you need professional help in more ways than one). Modern engines can sense when knocking occurs and change the timing of the spark (firing the cylinder earlier before detonation occurs) within certain limits. Taken the other way this is why a high-performance engine can eke more power out of high-octane fuel by moving the spark to a later, higher-compression part of the cycle, thus creating more power. But once you have enough octane to put the spark at the optimum point, ,more octane won't help matters,. An engine designed for 87-octane fuel should allow full compression without danger of detonation using that grade of fuel. Therefore, in fact, unless there is something wrong with your engine, ,using fuel with higher-than-recommended octane or adding octane booster is simply a waste of money. Doubly so with 2 bottles. So it's a waste, but is it going to hurt anything? Not in the short term; at any octane rating you could reasonably get with these additives the engine should still run and burn cleanly (that's why there's no restriction or mechanical lockout to prevent you from putting 93-octane fuel into 87-octane cars, as there is for Diesel fuel). If your car is designed for regular gas but has a slight knock it's reasonable to try the mid-grade or premium fuel first, as this is a more consistent and convenient way of getting higher octane than an additive. (octane booster is essentially a concentrated version of the additives that are already in most gas). It's also worth considering the ,environmental footprint, of that plastic bottle of ethanol and MTBF. Finally, if you are already buying premium fuel and find that you need to be adding this stuff over and above 93 octane fuel to make your car run right -- or if you think you need 2 bottles at a time to avoid a knock -- you have something wrong and are probably polluting and not going to pass inspection anyway.