Šta je snaga, a šta obrtni moment?

[dragvorl]

Evo interesantnog objasnjenja zasto dizeli imaju vise momenta:

Now let's discuss effective cylinder pressure on the power stroke of a diesel as compared to that in a gasoline engine. We've already mentioned that higher turbocharger boost raises the effective cylinder pressure, but let's look at what else comes into play. In "Understanding Today's Diesel" elsewhere on this site, the way fuel is introduced into the cylinder is thoroughly discussed for both gasoline and diesel engines. Gasoline engines mix the fuel with the air before it enters the cylinder, so when the intake valve closes, the power potential of that air and fuel charge is set. The timed spark ignites the mixture and cylinder pressure rises to a peak at roughly 15º after TDC. Because the combustion process takes time, combustion may or may not be complete by 15º after TDC depending on engine RPM, but for all practical purposes, we can say that the process of combustion is concluded early in the power stroke and that no more heating of the working fluid (the gases in the cylinder) occurs. This means the force acting on the piston top is highest at a time when the connecting rod has very little leverage on the crankshaft pin. As the crankshaft continues to rotate past TDC, the leverage the piston can exert increases, but the pressure on the piston top is dropping quickly. This, too, is discussed in the aforementioned article.

Once you envision when combustion occurs and the relationship between cylinder pressure and leverage on the crankshaft, it becomes obvious that if we could continue the burning process longer into the power stroke, additional cylinder pressure could be generated to push on the piston top as connecting rod-to-crankshaft angle improves for more leverage, and hence more torque. This is exactly what happens in a diesel. Because the fuel is injected into the cylinder after the intake valve is closed and the air is compressed, the length of the fuel injection pulse, called pulse width, can be extended well into the power stroke. This means the average effective cylinder pressure acting on the piston is higher in a diesel than in a comparably sized gasoline engine. The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels. And all of this taken together makes it apparent why diesel engines have to be built with such robust parts to withstand this high cylinder pressure and torque.

http://www.bankspower.com/techarticles/ ... uch-Torque

[snork]

Ali to važi samo za turbo dizele? Kod atmosferskih su i snaga i moment, koliko sam do sada mogao da vidim iz raznih specifikacija, uvek manji nego kod atmo benzinca iste zapremine. Kada nema dovoljno vazduha, nema svrhe ni produžavati ubrizgavanje goriva, jer neće svo sagoreti. Tako da, ako sam dobro razumeo, ovaj tekst objašnjava zašto turbo dizeli mogu imati veći max. moment od istih takvih (turbo) benzinaca?

[nikaragva]

Dobro, znaci zakljucak je da je dizel motor iste zapremine "snazniji" (odnosno ima veci moment) nego benzinski u tom smislu, da moze da okrene menjac, bez da mu se namestaju mega range zupcanici i 20 brzina
To valjda vodi ka jednostavnijem menjacu, pa i manjoj potrosnji.
Jer valjda je cij i bio da naprave motor koji ce povlaciti teret, bez komplikovanog menjaca, koji bi mu omogucavao da se zavrti na 5000o/min pa tek onda pokaze zube

Iz istog razloga su lokomotive uvek pokretane sa EM, kao i oni veliki kamioni u rudnicima, gde im gorivo sluzi samo da pokrene generator i naprave struju.

[dragvorl]

Ali to važi samo za turbo dizele? Kod atmosferskih su i snaga i moment, koliko sam do sada mogao da vidim iz raznih specifikacija, uvek manji nego kod atmo benzinca iste zapremine. Kada nema dovoljno vazduha, nema svrhe ni produžavati ubrizgavanje goriva, jer neće svo sagoreti. Tako da, ako sam dobro razumeo, ovaj tekst objašnjava zašto turbo dizeli mogu imati veći max. moment od istih takvih (turbo) benzinaca?

Ubrizgava se toliko goriva koliko moze da sagori. Dizel sagoreva sporije od benzina pa mu se ne moze samo odjednom grunuti gorivo. Dakle ako je manje goriva bice i manje energije.

Dobro, znaci zakljucak je da je dizel motor iste zapremine "snazniji" (odnosno ima veci moment) nego benzinski u tom smislu, da moze da okrene menjac, bez da mu se namestaju mega range zupcanici i 20 brzina
To valjda vodi ka jednostavnijem menjacu, pa i manjoj potrosnji.
Jer valjda je cij i bio da naprave motor koji ce povlaciti teret, bez komplikovanog menjaca, koji bi mu omogucavao da se zavrti na 5000o/min pa tek onda pokaze zube

Iz istog razloga su lokomotive uvek pokretane sa EM, kao i oni veliki kamioni u rudnicima, gde im gorivo sluzi samo da pokrene generator i naprave struju.

Ne bi to kod benzinca bio nikakav poseban menjac, samo bi prenosni odnosi bili namesteni tako da motor odrzavaju u idealnom opsegu obrtaja posle svake promene stepena prenosa, sto dizeli imaju vec ovako, ali to bi srkalo mnogo goriva kao sto je vec primeceno. Inace si u pravu da su u lokomotivama SUS motori samo pogon za generatore, a EM pokrece tockove. Uglavnom su SUS motori kod lokomotiva dvotaktni dizeli.

[snork]

Pa nije to zaključak, nije snažniji i nema veći moment. Samo ih dostiže na manjim obrtajima + troši manje goriva. U svakom slučaju, nisam kompetentan za tu materiju, pa dalju diskusiju prepuštam mašincima.

A dizel-električni pogon se koristi ne zato što bi menjač bio komplikovan, već zato što ga uopšte nije ni moguće napraviti kada snage pređu par MW (a da ne bude užasno glomazan i ima ogromne gubitke). Na primer, kod malih dizel šinskih vozila (npr. šinobusa), postoji klasičan menjač i kvačilo, kao kod autobusa i kamiona. Kod većih lokomotiva, zupčanika nema, a jedini prenos je hidrodinamička spojnica (torque convertor). Kod najvećih dizel lokomotiva, kakve možeš videti i kod nas (snage 4-5 MW), gubici - grejanje u mehanici ili hidraulici bi bili preveliki, pa dizel motor pokreće električni generator, a on napaja električni motor. Istu vrstu prenosa koriste i najveći kamioni - damperi, kao što su oni beloruski (bile su slike na starom forumu).

[nikaragva]

Pa ako ne mogu da ga naprave, znaci da je komplikovan Morali bi da vuku jos jednu lokomotivu koja bi u stvari bila menjac.

Jel zna neko sta je to u konstrukciji dizela sto mu omogucuje da za iste parametre ima veci obrtni moment, nebitno za obrtaje? Veca snaga goriva, pa mogu drugacije da postave poluge i naprave veci moment ili nesto drugo?

[dragvorl]

Eno ti napisano u prvom postu gore. Izmedju ostalog kompresija je veca, i pravilno si rekao, specificni sadrzaj energije u gorivu je veci. Takodje nacin na koji sagoreva gorivo je dugaciji. Dizel ima sagorevanje rasporedjeno na vecem opsegu, dok benzin izgori brzo i ne pritiska radilicu najvise u onim momentima kada poluga moze da razvije maksimalni moment.

[Dusan]

Ali to važi samo za turbo dizele? Kod atmosferskih su i snaga i moment, koliko sam do sada mogao da vidim iz raznih specifikacija, uvek manji nego kod atmo benzinca iste zapremine. Kada nema dovoljno vazduha, nema svrhe ni produžavati ubrizgavanje goriva, jer neće svo sagoreti. Tako da, ako sam dobro razumeo, ovaj tekst objašnjava zašto turbo dizeli mogu imati veći max. moment od istih takvih (turbo) benzinaca?

2.0 SDI nema ni 70 konja i bednih 140 Nm, dok npr atmo kod mene 2.0 FSI ima 150 i 200Nm.
A sad opet i ako pogledamo najjaci 2.0 TDI (170, 350Nm) i uporedimo sa najjacim 2.0 TSI (265, 350 Nm) obrtni je isti ali snaga se drsticno razlikuje.
(uzeo sam VAG motore jer su mi najlaksi za komparaciju)

EDIT: da dodam da jednostavno iako raspolazu istim obrtnim dizel ne moze da se vrti na 6000-7000 obrtaja i proizvede snagu.

[dragvorl]

Evo jedan po meni pametan post u vezi obrtnog momenta diesel vs benzin:

In theory, the diesel engine is actually less efficient than a gasoline engine due to the thermodynamic cycle by which it operate. But in practice a diesel engine is more efficient than a gasoline engine, the main reason is the use of a higher compression ratio, and at part load, the lack of throttling. In practice the thermal cycle of the diesel also look very much like the cycle of the gasoline engine, more toward heat release at constant volume than at constant pressure.

The modern diesel engine is direct injected and the combustion occur in two stages. When the fuel injection starts before TDC a small part of the fuel will vaporise due to the compression heat, and the longer the ignition delay is, the more fuel will have had time to vaporise. This fuel will mix with the air inside the cylinder and will burn rapidly when ignited. This is the premixed burning in a diesel engine and since the heat release rate (J/deg) is very high it will result in a pressure spike and the knocking engine sound of a diesel engine. When the load, but also engine speed is increased the ignition delay will become shorter and the pressure spike will disappear.
The second phase of diesel engine combustion occur with a diffusion flame. Unlike a direct injected gasoline engine the fuel injection isn't completed before the combustion starts, no, the injection continue during the burn. The injector nozzle inject liquid fuel, a small diesel usually have around 5 sprays per injector, which vaporise and oxidize when it comes into contact with air. If you haven't seen this on an in cylinder video, it looks like having 5 very small flame throwers in the cylinder. Inside the plume the mixture is rich, which cause the formation of soot, and at the outside the mixture is lean, causing the formation of nitrous oxides. This is shown in the picture below.

http://www.scielo.br/img/revistas/jbsms ... 5398f1.jpg

In a gasoline engine the combustion speed is dependant on the engine speed. Higher engine speed means more turbulence, which means a higher combustion speed. In the gasoline engine, the result is that the combustion can be done at about the same duration in crankshaft degrees independent on engine speed (only the ignition delay increase with speed). This is not the case with the diesel, where the duration of the combustion becomes longer and longer when the engine speed increases. The result is that the maximum cylinder pressure, pmax, occur later and later in the power stroke. With a gasoline engine you want pmax to occur at around 17 degrees after top dead center, and the ignition advance is set to achieve that if possible. If the ignition is retarded we get less torque, a lower efficiency and a higher exhaust temperature. The same thing will happen if we run a gasoline engine lean as that would result in a slower burn. This is also what is happening to the diesel at high engine speeds. So, the engine speed is kept down with diesels.

Diesel engines always operate with lean fuel mixtures, and the load is also controlled by adjusting the amount of fuel injected as opposed to air as with the gasoline engine. The diesel also have a smoke limit somewhere around lambda 1.3, use richer fuel mixtures than that and the engine will start to produce visible smoke while the exhaust temperature increase.
High injection pressures, as high as 2400 bar, are mainly used to prevent soot, important in itself but also important if an EGR system should be possible to use. Supercritical fuel injection is studied as a possible next step.

It is often said that diesels produce more torque than gasoline engines. They don't. Compare a diesel engine on equal grounds, that is, equal displacement and boost pressure, and they will produce about the same amount of torque. The diesel will however in such a comparison produce much less power. The reason diesels often produce more torque than a gasoline engine is either due to a greater displacement or a higher boost pressure. Since a diesel can't knock it can use very high boost pressures to compensate for what it lacks in engine speed. But high boost pressures combined with a high compression ratio also result in very high peak cylinder pressures. A naturally aspiranted engine producing 100 hp/l such as several BMW engine, see peak pressures in the order of 90 bar, a F1 engine 110 bar or so and a downsized turbo engine such as the VW TSI 120 bar. A diesel with 100 hp/l operate with peak pressures in the order or 190 bar, roughly twice that of the gasoline engine with similar specific output. For a production engine that translates into a roughly 20% weight handicap since the engine must be reinforced to handle this king of pressures.

Diesel fuel contain 42.5 MJ/kg and has a specific gravity of 820-845 kg/m3. Premium gasoline contain more energy, 43.5 MJ/kg, but has a lower density of 720-745 kg/m3. Overall, this gives diesel roughly 10% more energy ona volume basis. Scandinavian diesel, or more specifically, Swedish MK1 diesel have a reduced aromatics content to make the fuel cleaner, that result in a less dense fuel at 810 kg/m3 but it also has a higher energy content of 43.2 MJ/kg.

For a diesel engine to be possible in F1 which require a compact lightweight package, it would probably be forced to operate with a low compression ratio by diesel engine standards and with a very high boost pressure. Since the engine would be forced to produce the same output as the competition, but at a much lower engine speed, it would also put high demands on the drivetrain.

http://www.f1technical.net/forum/viewto ... 3&p=143453

[Xepoj87]

Super tekst koji ukratko objasnjava sustinu koja je nama laicima bitna

[dragvorl]

Evo po meni odlicnog objasnjenja.

[Laki021]

Ja cu samo jednu jer ipak, vozim Mercedes:

BMW ima 2.0 dizel da 184ks i 380 Nm

Mercedes ima 2.2 dizel sa 170ks i začuđujuće više Nm odnosno 400?


Pošto dosta ljudi ovde tvrdi da je obrtni moment bitniji za vožnju i da se on ustvari "vozi" zaključujem da je Mercedes zapravo "ultimate driving machine" a ne BMW:-):-):-):-)


Sent from my GT-I9300 using Tapatalk 2

Onda dosta ljudi ovde debelo gresi, jer smo vec u vise navrata na forumu u nekoliko tema dokazali da je snaga znatno bitnija od Nm...

[Xepoj87]

Off topic
Ehh,to tako samo prodavac automobila moze da kaze,da je jedno bitno,kao sto mi je danas jedan doticni na placu rekao,gde stoji fenomenalno lepa perla plava Alfa 147 sa nekim opakim sportskim paketom,restyle,2008.,150.000km 1.9 150ks,crni krov,felne,retrovizori... Mozda posle one narandzaste 147 GTA,najlepsa 147 koju sam ikada video,a kosta 5k e
Elem,zanesoh se ja u prisecanju Alfe

Rece covek: Ima 1.9 motor,150ks,to bukvalno ne moze da se vozi,cupa volan iz ruku... Drugar i ja smo se samo pogledali u fazonu: Sve nam je jasno
Pogotovo sto taj drugar vozi sa istim tim motorom Fiat Bravo-a


Elem,bitno je tu milion stvari,tezina,menjac,sistem proklizavanja,motor,Nm,Ks,CC,turbina,pogon,ovo i ono...

Tako da to koliko konja ima i koliko njutna mi je bespoentno bez konkretnih brojki kako su isti iskorisceni
Tu ja opet mislim da je Bmw u prednosti,sto bolje koristi ono sto ima
I to ima svoju cenu,kao i sve ostalo,pa ko sta voli,nek' izvoli

[Laki021]

Necu ponovo da pricam pricu i objasnjavam, ali cisto zbog zakona fizike i samog znacenja tih velicina snaga je bitnija od obrtnog momenta. Moment mozete nadoknaditi radom menjaca a snagu nikako. Jasno da je bitno jos mnogo stvari u celoj prici, ali generalno sta je bitnije KS ili Nm zapravo i nije neka dilema stvar je vrlo jasna. Cela zabuna i mit je nastao zbog velikog obrtnog momenta dizel motora i nacina na koji oni povlace, ali to je zapravo zbog bolje krive a ne veceg momenta...

[perfiko]

A da nije možda zbog oboje?