Cum discutia despre premiile obtinute de BMW degenerase clar intr-una OHV vs. OHC si cum mi-am dat seama ca eu unul nu intelegeam nimic am zis sa discutam mai pe larg aici... Desigur se va ajunge si al off-topic de genul Americane vs Europene sau Asiatice insa per total poate intelegem toti ce e cu toate acronimele acestea... Ce as dori este ca cei cu experienta in condusul masinilor cu diferite motoare sa ne aduca si ceva deatlii suplimentare vis-a-vis de feelingul la volan sau orice alte lucruri importante de stiut...
Pentru inceput as pune eu niste lucruri interesante pe care le-am gasit despre OHV, OHC...
1.
http://www.samarins.com/glossary/dohc.htmlOHV, OHC, SOHC and DOHC (twin cam) engine design
Automotive illustrated glossary
OHV engine design
4-cylinder 8 valves OHV engine
OHV means OverHead Valve - an engine design where the camshaft is installed inside the engine block and valves are operated through lifters, pushrods and rocker arms (an OHV engine also known as "Pushrod" engine). Although an OHV design is a bit outdated, it has been successfully used for decades. An OHV engine is very simple, has more compact size and proven to be durable.
Downsides: it's difficult to precisely control the valve timing at high rpm due to higher inertia caused by larger amount of valve train components (lifter-pushrod-rocker arm). Also it's very difficult to install more than 2 valves per cylinder or implement some latest technologies such as Variable Valve Timing - something that could be easily done in a DOHC engine.
OHC or SOHC engine
4-cylinder 8 valves SOHC engine
OHC in general means OverHead Cam while SOHC means Single OverHead Cam.
In the SOHC engine the camshaft is installed in the cylinder head and valves are operated either by the rocker arms or directly through the lifters (as in the picture).
The advantage is that valves are operated almost directly by the camshaft - easy to achieve the perfect timing at high rpm. Also it's possible to install three or four valves per cylinder
The disadvantage - an OHC engine requires a timing belt or chain with related components - more complex and more expensive design.
DOHC or Twin cam engine
4-cylinder 16 valves DOHC engine
DOHC or Double OverHead Cam - the setup used in many today's cars. Since it's possible to install multiple valves per cylinder and place intake valves on the opposite side from exhaust vales, DOHC engine can "breath" better meaning that it can produce more horsepowers with smaller engine volume. Compare: The 3.5-liter V6 DOHC engine of 2003 Nissan Pathfinder has 240 h.p, similar to 245 h.p of the 5.9-liter V8 OHV engine of 2003 Dodge Durango.
Pros: High efficiency, possible to install multiple valves per cylinder and adopt variable timing.
Cons: More complex and more expensive design.
2.
http://www.datsuns.com/Tech/ohv_vs_ohc.htmOHV vs SOHC vs DOHC and 4-valves/cyl
By Dave Lum 4/99
updated 1/01 (2v/cyl vs 4v/cyl)
blank50.gif (825 bytes) OHV (Overhead Valve) SOHC (Single Overhead camshaft), and DOHC (Double Overhead Camshaft) are all terms to describe the cylinder head layout of a modern piston engine.
OHV While all modern automotive piston engines have the valves arranged over the cylinders, OHV engines were named because earlier engines (like the 1930's and 40's) had valves beside the cylinders, not OVER them - so OHV was a a way to differentiate the OHV valve style from say, the Flathead engines. OHV engines have the camshaft below the cylinder head, and thus use lifters and pushrods to help actuate the valves that are in the cylinder head. Compared to OHC engines, they allow for better packaging, but are less efficient compared to OHC designs due to increased valvetrain mass. To open a valve, the camshaft pushes on a lifter, which pushes a pushrod, which pushes on a rocker am, which opens the valve. OHC engines don't have the weight of the pushrod to overcome. While the weight of a pushrod & lifter is seemingly insignificant, when you consider it can account for more than 15% of the valvetrain mass, and it has to open a valve up to 6000 times a minute (or more), it adds up to measurable difference. It's all about inertia - the less weight it has to move, the less energy is required to open the valve, and thus, there is more energy that can be transferred to the crankshaft - meaning more HP to the wheels.
SOHC (Single overhead camshaft) Sometimes referred to as just OHC. The camshaft is in the cylinder head, and the camshaft opens the valve either almost directly (with a shim between the cam lobe and valve stem), or by actuating on a rocker arm. In either case, it doesn't have the added mass of a pushrod. Most SOHC designs have just 2-valves/cylinder, but some manufacturers (like Honda) have SOHC engines that use a 4-valve/cylinder arrangement. SOHC engines have a camshaft in each cylinder head - so an SOHC V-8 actually has two camshafts
DOHC (Double overhead camshaft) A DOHC arrangement is normally used to take advantage of a 4-valve/cylinder arrangement. A typical DOHC design has two camshafts in each cylinder head. The benefits are the same as an SOHC engine, with the added breathing of 4valves/cyl (see below)
4-valves/cylinder. It's all about inertia and breathing here. The key to high specific output (meaning HP per liter, or cu. in.) is breathing and low pumping losses. In the '60's and 70's, to get more HP out of a given displacement a manufacturer would install bigger valves. HP comes from cramming more air and fuel into the cylinder, so a bigger "door" (valve) can let in a bigger air/fuel mixture per stroke . The problem with a bigger valve is that a bigger valve weighs more than a smaller one (assuming the same metallurgy), and this means to control the extra weight as it's flung open is the spring that closes the valve has to be stiffer. A stiffer valve spring (multiplied by the number of valves), means more energy is spent overcoming the valve pressure, thus partially offsetting the gains of a bigger valve. Another disadvantage of a big valve is that at lower RPM's, the intake charge has a lower velocity and low RPM torque and driveability. suffer. Enter a mulitivalve design. Two 30mm valves give more breathing area than a single 60mm one, and each valve weighs less than a 60mm one, as well. I can hear you saying "but TWO 30mm valves weigh more than a single 60mm one!". True, but for each cam lobe, there is less mass to overcome, and the smaller valves don't have to open as far nor do the valve springs have to have as much spring rate. A 4-valve design almost gives you something for nothing, as at lower RPM's the intake velocity is good for each port (they're small ports) so you get good low RPM torque and driveability., while at high RPM's there's sufficient flow (there's two ports) to make good HP. In practice, there
In layman's terms, a DOHC 4-valve/cylinder (or even 5v/cyl) increases HP without sacrificing low RPM torque and driveability. It effectively extends the torque curve. Below is a chart of two identical displacement engines, one is SOHC 2v/cyl, and the other is DOHC and 4v/cyl : See the graphs below (click on images to see full-screen view). For those real nit-picky, you can think of this as 4v/cyl vs 2v/ cyl, as I am aware there are 4v/ cyl SOHC engines)
Nissan VG30E vs VG30DE
midsohc-vs-dohc-tq.gif (3084 bytes) midsohc-vs-dohc-hp.gif (3612 bytes)
2v/cyl vs 4v/cyl - torque 2v/cyl vs 4v/cyl - horsepower
These engines are from the same manufacturer and the same engine family - probably as close as you can get for a direct 2v/cyl vs 4v/cyl comparison. Both are Nissan 3.0L EFI engines. Both engines are relatively stock (only changes are those required to fit them into a 510), and both engines are in the same type of car (early 510). Note that they are essentially identical in torque (one more than 8ft-lbs difference) until 4250rpm, where similarities end. The 4v/cyl torque curve extends another 2000rpm! Since horsepower is torque*rpm/5252, increasing the RPM and maintaining the torque means more HP. Look at the HP chart and you see what I mean. If you race there cars and force both cars to shift at 4250rpm, they will be dead even. This also means in normal driving, in traffic, you will feel no difference.
Those of us who like to rev our engines, however, get to feel the added benefit of breathing, and we get to stay in a given gear longer. So where the 2v/cyl guy here has to shift to 2nd at about 5500rpm, the 4v/cyl car stays in first for another 1250rpm. That means for 1250rpm, the car in first gear is handily out-accelerating the guy who had to shift to 2nd. Doesn't seem like much, but in 2nd and 3rd, you're in those gears longer, which means a longer time for the 4v/cyl guy to stay one gear lower, adding to his lead.
Another advantage of a DOHC design is it makes it easier to add variable valve timing. Not all DOHC designs implement this, as it adds cost, but with variable valve timing, the engine can dynamically adjust the intake and exhaust timing to give an even better spread of low RPM and high RPM power. Honda/Acura, BMW, and Nissan/Infiniti are some of the manufacturers that have variable valve timing on some of their DOHC engines.
The drawback of DOHC? Cost (more parts, more money), packaging (the cylinder head is bigger due to all the cams and valves up there), and maintenance (you have to adjust twice as many valves).
3.
http://en.wikipedia.org/wiki/DOHC#Dual_overhead_camshaft4.
http://en.wikipedia.org/wiki/Pushrod5.
http://en.wikipedia.org/wiki/Naturally-aspirated_engine
Acest mesaj a fost modificat de akajohn: 30th January 2008, 17:26
OHV, OHC, Valvetronic, VTEC, TFSI si alte chestii de genul acesta, Sufletul masinii e motorul. Hai sa stim mai multe despre el.
30th January 2008, 17:23
