Wikipedia:Reference desk/Archives/Science/2017 February 8
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February 8[edit]
Why does the Diesel cycle assume a combustion at constant pressure?[edit]
For gasoline engines, the Otto cycle assumes combustion at constant volume, which can be justified (kind of) by "combustion is quick, so the piston hardly moves during it and the combustion chamber volume does not change". That is clearly an approximation since the ignition spark and subsequent combustion does not all occur at top dead center. Why is another approximation used in diesel engines? TigraanClick here to contact me 14:25, 8 February 2017 (UTC)
- I think it's because the fuel is injected after compression, and as such the primary compression and volume change has happened prior to ignition. I came to this conclusion by reading the intro to diesel cycle about five times. Diesel engine supports this, saying "At or slightly before 2 (TDC) fuel is injected and burns in the compressed hot air." -- indicating to me that the combustion is happening during (short!) period of relatively little change in pressure. Hopefully we can get some better refs, but I think the reason is at least weakly supported by our articles. SemanticMantis (talk) 15:12, 8 February 2017 (UTC)
- You are mistaken. The instantaneous combustion will lead to the instantaneous temperature increase and therefore instantaneous pressure increase – this happens in the Otto cycle. In diesels the reverse is true – the combustion is a relatively slow process as the injection of the fuel takes some time and this allows the piston to move thus maintaining constant pressure. Ruslik_Zero 18:01, 8 February 2017 (UTC)
- It sounds like you're saying the same thing as I did, but explaining it better. Can you tell me which part of what I wrote is wrong? I'm ready and willing to be wrong I'd just like to know how, so that I can learn :) SemanticMantis (talk) 20:59, 8 February 2017 (UTC)
- Your explanation seems to assume that there is "a short period of constancy" around TDC, and that a Diesel engine burns all its fuel then.
- The 'constancy' though is that of constant volume. not pressure. The piston slows down around TDC, so the volume changes little. This is part of the Otto cycle. What the pressure does though is dependent on how much heat (i.e. burned fuel) is added.
- The injection timing of a Diesel engine has varied a lot over the last hundred years. The original slow-speed Diesel began injection around TDC (only then would it be at ignition temperature) and continued during the early part of the power stroke, giving an approximately constant pressure. It might inject for maybe 30º after TDC - about 1/8 of the stroke (although I don't have many timing or indicator diagrams to hand for early slow-speed Diesels). Note also (a clarification from Diesel's 1892 patent to his 1895 patent) that the compression doesn't need to achieve the combustion temperature, merely the ignition temperature - the temperature and pressure, once ignition is achieved, is due to combustion, not compression.
- Diesels since 1930 are using the dual cycle. Injection begins maybe 15º BTDC (for an indirect injection engine) and there is a constant volume sudden pressure rise around TDC (as the charge suddenly ignites, after the fuel has already been entering), as for the Otto, with injection ceasing around TDC (under typical working load and fuel demand). Pressure then remains constant as the piston moves downwards while this fuel is burned, but not for as long as the original Diesel cycle. Andy Dingley (talk) 13:20, 9 February 2017 (UTC)
- User:Andy Dingley Thanks, that helps. So is it correct to say that the combustion in diesel is generally carried out in an increasing volume, but that rate of increase is designed to keep pressure relatively constant? SemanticMantis (talk) 16:19, 9 February 2017 (UTC)
- The volume is constrained, as it's set by the piston position alone, but the pressure can be controlled (by controlling the fuel input, thus the heat added) to maintain whatever pressure curve is desired - in the diesel, that's a constant pressure. Andy Dingley (talk) 16:45, 9 February 2017 (UTC)
- But apparently all of us lesser minds, "don't know about the history and development of compression ignition engines." Let alone about the Bosh process for braising injector pumps. Andy Dingley (talk) 18:18, 9 February 2017 (UTC)
- User:Andy Dingley Thanks, that helps. So is it correct to say that the combustion in diesel is generally carried out in an increasing volume, but that rate of increase is designed to keep pressure relatively constant? SemanticMantis (talk) 16:19, 9 February 2017 (UTC)
- It sounds like you're saying the same thing as I did, but explaining it better. Can you tell me which part of what I wrote is wrong? I'm ready and willing to be wrong I'd just like to know how, so that I can learn :) SemanticMantis (talk) 20:59, 8 February 2017 (UTC)
- You are mistaken. The instantaneous combustion will lead to the instantaneous temperature increase and therefore instantaneous pressure increase – this happens in the Otto cycle. In diesels the reverse is true – the combustion is a relatively slow process as the injection of the fuel takes some time and this allows the piston to move thus maintaining constant pressure. Ruslik_Zero 18:01, 8 February 2017 (UTC)
- This was part of Diesel's goal in inventing the Diesel cycle. It doesn't have constant pressure by accident, he designed the cycle so that combustion would take place under those conditions.
- Diesel wanted an efficient, and ideally powerful, engine capable of running on a cheap, simple fuel. He considered powdered coal, also early biodiesels such as peanut oil. Combustion chemistry was a new subject at the time and poorly understood, although thermodynamics was already quite advanced. The Otto engine required a complicated fuel prepared by partial distillation; this expensive fuel might be justified for a travelling lightweight car, but not for driving a factory. Before the 1930s development in petrol fuel chemistry, these fuels were also a serious limit on the performance of Otto engines. See the last two events of the Schneider Trophy.
- Diesel knew from thermodynamics and the work of Carnot that an efficient engine would need a high peak pressure, but also that a "fuller" indicator diagram would also help, rather than the two close-spaced isentropic curves of the Otto: this isn't a theoretical limit, but a practical one. The greater their separation, the easier to make an engine that could extract useful amounts of work without needing technically unachievable peak pressures.
- He also knew that short combustion times were beyond the chemical limits of cheap fuels. A sustained combustion phase would allow slower combustion. It also permitted the idea of continuously fuelling by injection, rather than a carburetted mixture supplied entirely during the inlet phase. Which allowed further advantages for a developed engine of permitting power control by controlling the fuel, rather than the mixture, avoiding the throttle valve and maintaining the same compression cycle at both high and low power outputs (both thermodynamically advantageous).
- These two goals gave Diesel the basis of his cycle and initial engine. Compression would be applied to an efficiently high pressure and the self-ignition temperature, then fuel would begin to be injected, at a rate sufficient to maintain constant pressure during the initial movement of the piston (some early air-blast engines governed their injection rate according to this pressure). The timing of ceasing to inject fuel would then control power, also allowing enough time with the piston still descending in order to allow the work done on the piston during the expansion phase of the cycle to represent the output work of the engine. Thus Diesel's initial air blast slow-speed Diesel engine.
- The engines are large, mostly because they need a long stroke and long unfuelled stroke to give enough expansion to extract a useful amount of the work. Eventually this limitation on the Diesel ends up with engines like the turbo-compound engine and the turbocharger, which allow further useful work to be extracted from high pressure exhaust gasses, even after they've left the cylinder.
- Around 1930, Ricardo achieves the first real high-speed diesel engines. These use his knowledge of combustion dynamics and inlet charge flow to improve the combustion sufficiently to allow a much faster combustion (of the same fuels), thus allowing more expansion (and work done) in a briefer power stroke. Injection now becomes 'solid' injection, i.e. a stream of pure liquid, sprayed in at extremely high pressure through an atomising nozzle. As Ricardo famously described it, if he couldn't make the fuel meet the air any faster, he would make more of the air reach the fuel spray by swirling it around rapidly, initially in a separate combustion chamber. Diesels from this age begin to use a dual cycle, rather than the pure Diesel cycle of before, regaining characteristics of the Otto in that their heat is added during conditions somewhere between constant pressure and constant volume. Andy Dingley (talk) 15:31, 8 February 2017 (UTC)
- Diesel specified his cycle to get around the existing patents of the time. However, it was so inefficient that it was quickly abandoned by engine manufactures. --Aspro (talk) 16:42, 8 February 2017 (UTC)
- Really? Andy Dingley (talk) 13:20, 9 February 2017 (UTC)
- Wait, what? Diesel engines have the highest fuel efficiency of any internal combustion engine of comparable size and power. Can you explain what you mean? Also, MANY engine manufacturers still produce diesel engines. Because of the fuel efficiency and thermal efficiency advantages, diesel engines have long been the favored engine type for most large vehicle applications, like tractor-trailers and locomotives and large ships. --Jayron32 14:39, 9 February 2017 (UTC)
- OK so you don't know about the history and development of compression ignition engines. The OP's question was about the Diesel Cycle. Modern compression ignition engines haven't use the Diesel's Cycle for many a year but diesel has become the geriatric term for compression ignition engines. Diesel licensed his name to be used for marine engines. They made engines under the brand name Diesel. Bit like sports man/personalities today endorsing a product in return for oodles of money. Later, C.A.V in Acton, London formed a partnership with Bosh in the 1930's to create the modern compression ignition engines which achieved a greater efficiency and economy than the Otto Cycle due to both the higher compression ratio and by reverting back to Wright's original patent of injecting fuel 4 deg before TDC. No mean feat, as it require pumping fuel in at about 450 atm's with cylinder pressure peaking at around 500. In order for the fuel lines to the injectors to withstand these pressure they have to be constructed by winding strips of steel in a helix and braising them into a tube (if you doubt me slice a tube, polish it, then look under the microscope – you'll see the braze). Rudolf’s engines blew up under these conditions. Back then only Britain and Sweden had these engineering and metallurgical skills so Bosh came to Britain. Later, methane compression ignition engines achieved the highest efficiency of all as the compression ratio is about 22:1. After the WW2 C.A.V. licensed Bosh to continue manufacturing fuel injection equipment. That is why all Bosh in-line- pumps and injectors of that era are identical and interchangeable with C.A.V. equipment. It is no more than a quirk of the vernacular that people don't refer to their gasoline/petrol cars as Otto's but those with oil ignition engines as diesel's. They really should be named Wright Engines if one want to be pedantic enough to be historically accurate.--Aspro (talk) 17:06, 9 February 2017 (UTC)
- Wow! That's a lot of names, dates, and specific claims! Have you got a reliable source to cite for any interested readers who want to study the topic in greater detail? I've got a stack of books on engines, and they all describe "diesel engines" in a pretty consistent fashion...
- If your library or institution provides access, A History of Automotive Engine Development is published by the Society of Automotive Engineers: it describes the technical and business history of several engine cycles, including the Diesel engine. For the really interested readers who want to find a "second opinion," the engineering history section of the American Society of Mechanical Engineers links to a whole library of free- and non-free historical publications. I don't believe there is any strong reason to disbelieve the standard narrative of history, at least not in this case.
- Nimur (talk) 17:56, 9 February 2017 (UTC
- OK so you don't know about the history and development of compression ignition engines. The OP's question was about the Diesel Cycle. Modern compression ignition engines haven't use the Diesel's Cycle for many a year but diesel has become the geriatric term for compression ignition engines. Diesel licensed his name to be used for marine engines. They made engines under the brand name Diesel. Bit like sports man/personalities today endorsing a product in return for oodles of money. Later, C.A.V in Acton, London formed a partnership with Bosh in the 1930's to create the modern compression ignition engines which achieved a greater efficiency and economy than the Otto Cycle due to both the higher compression ratio and by reverting back to Wright's original patent of injecting fuel 4 deg before TDC. No mean feat, as it require pumping fuel in at about 450 atm's with cylinder pressure peaking at around 500. In order for the fuel lines to the injectors to withstand these pressure they have to be constructed by winding strips of steel in a helix and braising them into a tube (if you doubt me slice a tube, polish it, then look under the microscope – you'll see the braze). Rudolf’s engines blew up under these conditions. Back then only Britain and Sweden had these engineering and metallurgical skills so Bosh came to Britain. Later, methane compression ignition engines achieved the highest efficiency of all as the compression ratio is about 22:1. After the WW2 C.A.V. licensed Bosh to continue manufacturing fuel injection equipment. That is why all Bosh in-line- pumps and injectors of that era are identical and interchangeable with C.A.V. equipment. It is no more than a quirk of the vernacular that people don't refer to their gasoline/petrol cars as Otto's but those with oil ignition engines as diesel's. They really should be named Wright Engines if one want to be pedantic enough to be historically accurate.--Aspro (talk) 17:06, 9 February 2017 (UTC)
- The best answer I can give you, is when I was an instrumentation engineer I designed and made traducers to measure engine performance. Before we could submit a patent we had to do a previous patent search (and in Europe we have to be far more exacting than in the US). Never thought back then, that these patent searches may come in useful 40 years later (to post on WP), so did not make any personal notes other than those I was employed to do. Yet, it is all still up there in my brain. So this is why I can come out with a “lot of names, dates, and specific claims” Oh. If only my foresight of the 1970's had the 20/20 vision of hind sight now, I would have taken notes to be able to provide you all with the references – and in detail. --Aspro (talk) 21:42, 9 February 2017 (UTC)
- Aspro, yet again WP:CIVIL makes giving you a fully descriptive response difficult.
- You have something of a point that (as has already been discussed) Diesel's original cycle isn't the one used today in (high-speed) diesel engines. But that's about the only thing you're any more than vaguely correct over. Also, take a look at today's most efficient engines, the slow-speed marine Diesel engines, and look at their indicator diagrams - they're still using Diesel's work, and are much closer to his idealised and original cycle than to other engines. Andy Dingley (talk) 18:16, 9 February 2017 (UTC)
- Thanks to Nimur for links supporting the historical accuracy. We can all now delve deeper. The link goes to: Marine heavy-oil engine installation practice and development possibilities Published: 1920-01-01. They talk of diesel-type engines and not of engines that employ the Diesel Cycle (which was already losing favour) on marine engines. Note: Marine engines manufactured by M.A.N using the diesel brand name. The article go on to suggest that they where cheaper than steam powered vessels. Yes, they where, but ships with engines which were powered by the original Wright Cycle engines where even cheaper to run – it is just pure economics that the Diesel Cycle was abandoned. So one should now see the confusion here. The first M.A.N. engines (more efficient than steam) where branded Diesel for maritime employment to get around the current patents (they were also too big and heavy to compete with land vehicles using the Otto Cycle). The term somehow stuck though in the vernacular; just like some people today refer to using their Dyson, Vax, etc., for hovering the floor. The indicator diagrams show the Wright Cycle, i.e. fuel injection before TDC and not the patented Diesel Cycle. That is the very point I'm pointing out to you and any other readers which Nimur's link supports.--Aspro (talk) 21:00, 9 February 2017 (UTC)
Are devices safe after a blackout?[edit]
When the lights go out and come back, could a connected and previously running device be damaged?94.143.79.160 (talk) 23:11, 8 February 2017 (UTC)
- Depends on the device.
- A hot machine with a cooling fan might overheat if the fans stop, even if the heater stops at the same time. This isn't too far from what happened at Chernobyl. This is rare though.
- In industrial workshops, most machinery will be controlled by a "No Volt Release" switch. This is a pushbutton switch, one for on and one for off. It's guaranteed that if the power fails, or if the machine cuts out from some temporary fault (these switches often act as overload detectors too) then the machine will not restart automatically once the power returns, or the circuit breaker is reset. These switches use a relay or contactor internally, and two low-power pushbuttons. This is also convenient for controlling a large machine from a small switch, or for providing multiple reliable off buttons around the machine. Such switches are less common in the US, as less reliable and less safe semi-mechanical switches are used instead - these often have to be replaced if US machinery is installed in a European workshop.
- If machinery must essentially start automatically, or if it's controlled remotely, there are often rules that it must be placed behind guards and that its control equipment labelled with the warning sign shown. Andy Dingley (talk) 23:31, 8 February 2017 (UTC)
- Andy Dingley's answer refers to the effects of blackout itself. Note however there can also be complications occuring before or sometimes after blackouts that could damage devices (the later only likely if they autorestart). In some cases, a blackout could be proceeded by a power surge. This could damage various devices if they have no protection. I know someone who lost IIRC a garage door opener (it was on but not running) and one or two other things after a transmission powerline outside their house suddenly collapsed. I myself have experienced two brownouts (by which I mean a significantly reduce voltage not blackouts in small areas or one or two phases being off as the term is sometimes used). In one case I lost a computer power supply, the computer was on at the time but quickly went off. (It was a decent brand too, Enermax but lacked an autovoltage instead was selectable 115/230V. However it didn't take anything with it, as some extremely crappy power supplies are known to do.) If the brownouts are caused by some fault, it's likely there may be a blackout while the fault is fixed. Again protection like an automatic voltage regulator will significantly reduce the risk of damage (and also potentially alow the device to function during the brownout. (An uninterruptable power supply will also generally help, that helps with the blackout too.) If you're around, you may notice the brownout due the the effect on motors (fans etc) and many lights (although this is getting complicated with LED lights etc) and it may be wise to turn off devices if it appears to be significant and you're not sure how well they'll cope. Note most likely the devices will be safe even if damaged and non functional. Nil Einne (talk) 23:54, 8 February 2017 (UTC)
- Yes, for instance, see Southern California Edison's outage tips, which advise shutting off and unplugging devices to protect them. --47.138.163.230 (talk) 06:35, 9 February 2017 (UTC)
- But that's not to protect the device, that's to avoid massive startup surges synchronised across large parts of the network. Andy Dingley (talk) 12:03, 9 February 2017 (UTC)
- What are you basing that claim on? The page says, "You can help prevent damage to your electronics and appliances by unplugging them…" --47.138.163.230 (talk) 22:38, 10 February 2017 (UTC)
- But that's not to protect the device, that's to avoid massive startup surges synchronised across large parts of the network. Andy Dingley (talk) 12:03, 9 February 2017 (UTC)
- Yes, for instance, see Southern California Edison's outage tips, which advise shutting off and unplugging devices to protect them. --47.138.163.230 (talk) 06:35, 9 February 2017 (UTC)
- Brownouts tend to be the cause of more damage than either surges or blackouts. There's normally protection against surges and devices will be tested that nothing goes wrong if the power suddenly fails. But brownouts can do nasty things which weren't anticipated properly if they go on for a while. Dmcq (talk) 13:29, 9 February 2017 (UTC)
- Andy Dingley's answer refers to the effects of blackout itself. Note however there can also be complications occuring before or sometimes after blackouts that could damage devices (the later only likely if they autorestart). In some cases, a blackout could be proceeded by a power surge. This could damage various devices if they have no protection. I know someone who lost IIRC a garage door opener (it was on but not running) and one or two other things after a transmission powerline outside their house suddenly collapsed. I myself have experienced two brownouts (by which I mean a significantly reduce voltage not blackouts in small areas or one or two phases being off as the term is sometimes used). In one case I lost a computer power supply, the computer was on at the time but quickly went off. (It was a decent brand too, Enermax but lacked an autovoltage instead was selectable 115/230V. However it didn't take anything with it, as some extremely crappy power supplies are known to do.) If the brownouts are caused by some fault, it's likely there may be a blackout while the fault is fixed. Again protection like an automatic voltage regulator will significantly reduce the risk of damage (and also potentially alow the device to function during the brownout. (An uninterruptable power supply will also generally help, that helps with the blackout too.) If you're around, you may notice the brownout due the the effect on motors (fans etc) and many lights (although this is getting complicated with LED lights etc) and it may be wise to turn off devices if it appears to be significant and you're not sure how well they'll cope. Note most likely the devices will be safe even if damaged and non functional. Nil Einne (talk) 23:54, 8 February 2017 (UTC)
- We have a (really poor in my opinion) Dangerous restart article, and No-volt release is a redirect to a section of it. DMacks (talk) 16:41, 9 February 2017 (UTC)
- That's why I didn't link to it, it's that bad. Also NVR (In UK practice this is capitalised and not hyphenated) is a sufficiently different device from a US n-vr switch that our HSE and electrical regulations identify them separately. Andy Dingley (talk) 19:08, 9 February 2017 (UTC)
- Also note that if a device (which requires monitoring when in use) comes back on when no longer monitored, that can cause a problem. For example, a clothes iron. Hopefully such devices would require hitting a reset switch once power is restored, but some do not. StuRat (talk) 16:48, 9 February 2017 (UTC)
- It's not unusual in commercial premises that such devices are either forbidden, must have NVR switches added, or must have an identifying plug (my workshop uses redplugs). This is partly so that they can be identified and switched off manually, also so that they're not left on at night. Andy Dingley (talk) 19:08, 9 February 2017 (UTC)