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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet badly tuned or just underpowered?
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<blockquote data-quote="clevtrev" data-source="post: 22650" data-attributes="member: 187"><p>In the same vein, an engine running at full throttle may be happy due to a </p><p>rich WOT air/fuel ratio. Throttling back to part throttle the mixture may be </p><p>leaner and detonation may now occur. Bingo, the piston overheats and scuffs, </p><p>the engine fails but the postmortem doesn't consider detonation because the </p><p>the failure didn't happen at WOT.</p><p>I want to reinforce the fact that the detonation pressure spike is very </p><p>brief and that it occurs after the spark plug normally fires. In most cases </p><p>that will be well after ATDC, when the piston is moving down. You have high </p><p>pressure in the chamber anyway with the burn. The pressure is pushing the </p><p>piston like it's supposed to, and superimposed on that you get a brief spike </p><p>that rings the engine.</p><p>CAUSES</p><p>Detonation is influenced by chamber design (shape, size, geometry, plug </p><p>location), compression ratio, engine timing, mixture temperature, cylinder </p><p>pressure and fuel octane rating. Too much spark advance ignites the burn too </p><p>soon so that it increases the pressure too greatly and the end gas </p><p>spontaneously combusts. Backing off the spark timing will stop the </p><p>detonation. The octane rating of the fuel is really nothing magic. Octane is </p><p>the ability to resist detonation. It is determined empirically in a special </p><p>running test engine where you run the fuel, determine the compression ratio </p><p>that it detonates at and compare that to a standard fuel, That's the octane </p><p>rating of the fuel. A fuel can have a variety of additives or have higher </p><p>octane quality. For instance, alcohol as fuel has a much better octane </p><p>rating just because it cools the mixture significantly due to the extra </p><p>amount of liquid being used. If the fuel you got was of a lower octane </p><p>rating than that demanded by the engine's compression ratio and spark </p><p>advance detonation could result and cause the types of failures previously </p><p>discussed.</p><p>Production engines are optimized for the type or grade of fuel that the </p><p>marketplace desires or offers. Engine designers use the term called MBT ( </p><p>Minimum spark for Best Torque) for efficiency and maximum power; it is </p><p>desirable to operate at MBT at all times. For example, let's pick a specific </p><p>engine operating point, 4000 RPM, WOT, 98 kPa MAP. At that operating point </p><p>with the engine on the dynamometer and using non-knocking fuel, we adjust </p><p>the spark advance. There is going to be a point where the power is the </p><p>greatest. Less spark than that, the power falls off, more spark advance than </p><p>that, you don't get any additional power.</p><p>Now our engine was initially designed for premium fuel and was calibrated </p><p>for 20 degrees of spark advance. Suppose we put regular fuel in the engine </p><p>and it spark knocks at 20 degrees? We back off the timing down to 10 degrees </p><p>to get the detonation to stop. It doesn't detonate any more, but with 10 </p><p>degrees of spark retard, the engine is not optimized anymore. The engine now </p><p>suffers about a 5-6 percent loss in torque output. That's an unacceptable </p><p>situation. To optimize for regular fuel engine designers will lower the </p><p>compression ratio to allow an increase in the spark advance to MBT. The </p><p>result, typically, is only a 1-2 percent torque loss by lowering the </p><p>compression. This is a better trade-off. Engine test data determines how </p><p>much compression an engine can have and run at the optimum spark advance.</p><p>For emphasis, the design compression ratio is adjusted to maximize </p><p>efficiency/power on the available fuel. Many times in the aftermarket the </p><p>opposite occurs. A compression ratio is "picked" and the end user tries to </p><p>find good enough fuel and/or retards the spark to live with the </p><p>situation...or suffers engine damage due to detonation.</p><p>Another thing you can do is increase the burn rate of the combustion </p><p>chamber. That is why with modem engines you hear about fast burn chambers or </p><p>quick burn chambers. The goal is the faster you can make the chamber burn, </p><p>the more tolerant to detonation it is. It is a very simple phenomenon, the </p><p>faster it burns, the quicker the burn is completed, the less time the end </p><p>gas has to detonate. If it can't sit there and soak up heat and have the </p><p>pressure act upon it, it can't detonate.</p><p>If, however, you have a chamber design that burns very slowly, like a </p><p>mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC. </p><p>Because the optimum 14 degrees after top dead center (LPP) hasn't changed </p><p>the chamber has far more opportunity to detonate as it is being acted upon </p><p>by heat and pressure. If we have a fast burn chamber, with 15 degrees of </p><p>spark advance, we've reduced our window for detonation to occur </p><p>considerably. It's a mechanical phenomenon. That's one of the goals of </p><p>having a fast burn chamber because it is resistant to detonation.</p><p>There are other advantages too, because the faster the chamber burns, the </p><p>less spark advance you need. The less time pistons have to act against the </p><p>pressure build up, the air pump becomes more efficient. Pumping losses are </p><p>minimized. In other words, as the piston moves towards top dead center </p><p>compression of the fuel/air mixture increases. If you light the fire at 38 </p><p>degrees before top dead center, the piston acts against that pressure for 38 </p><p>degrees. If you light the spark 20 degrees before top dead center, it's only </p><p>acting against it for 20. The engine becomes more mechanically efficient.</p><p>There are a lot of reasons forfast burn chambers but one nice thing about </p><p>them is that they become more resistant to detonation. A real world example </p><p>is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1 </p><p>compression ratio. It was a premium fuel engine. For the 2000 model year, we </p><p>revised the combustion chamber, achieved faster bum. We designed it to </p><p>operate on regular fuel and we only had to lower the compression ratio .3 to </p><p>only 10:1 to make it work. Normally, on a given engine (if you didn't change </p><p>the combustion chamber design) to go from premium to regular fuel, it will </p><p>typically drop one point in compression ratio: With our example, you would </p><p>expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1 </p><p>in order to work on regular. Because of the faster burn chamber, we only had </p><p>to drop to 10:1. The 10:1 compression ratio still has very high compression </p><p>with attendant high mechanical efficiency and yet we can operate it at </p><p>optimum spark advance on regular fuel. That is one example of spark advance </p><p>in terms of technology. A lot of that was achieved through computational </p><p>fluid dynamics analysis of the combustion chamber to improve the swirl and </p><p>tumble and the mixture motion in the chamber to enhance the bum rate.</p></blockquote><p></p>
[QUOTE="clevtrev, post: 22650, member: 187"] In the same vein, an engine running at full throttle may be happy due to a rich WOT air/fuel ratio. Throttling back to part throttle the mixture may be leaner and detonation may now occur. Bingo, the piston overheats and scuffs, the engine fails but the postmortem doesn't consider detonation because the the failure didn't happen at WOT. I want to reinforce the fact that the detonation pressure spike is very brief and that it occurs after the spark plug normally fires. In most cases that will be well after ATDC, when the piston is moving down. You have high pressure in the chamber anyway with the burn. The pressure is pushing the piston like it's supposed to, and superimposed on that you get a brief spike that rings the engine. CAUSES Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel. A fuel can have a variety of additives or have higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance detonation could result and cause the types of failures previously discussed. Production engines are optimized for the type or grade of fuel that the marketplace desires or offers. Engine designers use the term called MBT ( Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. For example, let's pick a specific engine operating point, 4000 RPM, WOT, 98 kPa MAP. At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power. Now our engine was initially designed for premium fuel and was calibrated for 20 degrees of spark advance. Suppose we put regular fuel in the engine and it spark knocks at 20 degrees? We back off the timing down to 10 degrees to get the detonation to stop. It doesn't detonate any more, but with 10 degrees of spark retard, the engine is not optimized anymore. The engine now suffers about a 5-6 percent loss in torque output. That's an unacceptable situation. To optimize for regular fuel engine designers will lower the compression ratio to allow an increase in the spark advance to MBT. The result, typically, is only a 1-2 percent torque loss by lowering the compression. This is a better trade-off. Engine test data determines how much compression an engine can have and run at the optimum spark advance. For emphasis, the design compression ratio is adjusted to maximize efficiency/power on the available fuel. Many times in the aftermarket the opposite occurs. A compression ratio is "picked" and the end user tries to find good enough fuel and/or retards the spark to live with the situation...or suffers engine damage due to detonation. Another thing you can do is increase the burn rate of the combustion chamber. That is why with modem engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate. If, however, you have a chamber design that burns very slowly, like a mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC. Because the optimum 14 degrees after top dead center (LPP) hasn't changed the chamber has far more opportunity to detonate as it is being acted upon by heat and pressure. If we have a fast burn chamber, with 15 degrees of spark advance, we've reduced our window for detonation to occur considerably. It's a mechanical phenomenon. That's one of the goals of having a fast burn chamber because it is resistant to detonation. There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build up, the air pump becomes more efficient. Pumping losses are minimized. In other words, as the piston moves towards top dead center compression of the fuel/air mixture increases. If you light the fire at 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If you light the spark 20 degrees before top dead center, it's only acting against it for 20. The engine becomes more mechanically efficient. There are a lot of reasons forfast burn chambers but one nice thing about them is that they become more resistant to detonation. A real world example is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1 compression ratio. It was a premium fuel engine. For the 2000 model year, we revised the combustion chamber, achieved faster bum. We designed it to operate on regular fuel and we only had to lower the compression ratio .3 to only 10:1 to make it work. Normally, on a given engine (if you didn't change the combustion chamber design) to go from premium to regular fuel, it will typically drop one point in compression ratio: With our example, you would expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1 in order to work on regular. Because of the faster burn chamber, we only had to drop to 10:1. The 10:1 compression ratio still has very high compression with attendant high mechanical efficiency and yet we can operate it at optimum spark advance on regular fuel. That is one example of spark advance in terms of technology. A lot of that was achieved through computational fluid dynamics analysis of the combustion chamber to improve the swirl and tumble and the mixture motion in the chamber to enhance the bum rate. [/QUOTE]
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