Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
Rimmed duallobe... lol! I like it, might get one for the missus...
Some diesels are around the square (bore to stroke) and have little problem.
BMW are using in their new diesels (84mm bore and 90mm stroke) which isn't that under square.
High compression ratio really becomes a problem on really short stroke engines, due to the need to get the piston out of the way to allow the valves to open.
F1 have around 11:1 with a 40mm stroke. I know by my designing that this is about the maximum limit. Below 40mm you cannot get the piston to valve clearance, and if you go over 11:1 you are limited by valve angle, ie. above 11:1 with a 40mm stroke, the valves have to start going to a verticle movement and the flow of the ports and chamber shape suffers.
I know that most diesels have almost verticle valves and mostly have a chamber for the injector to operate properly. So in a larger bore engine the piston to head clearance is an issue, which limits compression ratio. At this point reducing the bore while maintaining the same piston to head clearance raises the compression ratio. The sacrifice is the shape and angle of the port, although turbos make it less of an issue.
Last edited by revetec; 02-27-2008 at 04:06 PM.
I sighted a dual lobe cam in a figure 8 type shape with a rim on the side to retain it. The patent also had a variations of a 90deg X with 4 pistons held together with cables from one piston to another at 90degrees and a dual counter rotating twin lobed variant. I will try and locate the patent number for you.
BTW. there are over 300 patents on cam engines which I sighted prior to my application, two of them now are ours.
I never said that diesel engines have to be or are all severly under square. I said that it is easier to achieve the high compression ratios (over 20:1) that they need if they maintain a relatively undersquare design.
Exactly and modern diesels are running over 20:1 CRs and they normally have valve angles close to 0deg (or 180deg if you prefer) the combustion chamber is normally part of the piston (which is an effective way of "reducing" the effective bore at TDC) and the flow through the ports is compromised (one of the reasons Diesels require Turbos and have problems producing high RPM power).Originally Posted by revetec
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
My understanding is that Diesels don't rev as high because of the combustion physics of the fuel. It simply takes longer to vaporise, plus with the added weight needed for dealing with higher pressures, increases reciprocating mass.
Last edited by revetec; 02-27-2008 at 07:20 PM.
The upper rev limit of diesel engines is held back by piston speed. Diesel engines are designed to work with constant pressure combustion (as opposed to constant volume combustion). This fact combined with the relatively low flame front speed of diesel leads to a maximum piston speed significantly lower than petrol engines. They are also strangled at higher RPM by poor port shape which again holds back maximum piston speed. Because the maximum piston speed is set and because diesel engines are made with relatively long strokes they can not achieve high RPM operation.
I have seen a model diesel engine that was able to rev past 10000RPM but that is because it had a stroke of only 10mm and was carburetted (meaning it was running closer to constant volume combustion then constant pressure).
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
I'll first state that Diesel really isn't my area of expertise, even though I have some knowledge in this area.
I don't think that a constant higher pressure would be a major hindrance to high RPMs, although I haven't given it enough thought and research.
This would be a good topic to research and discuss in great detail.
What is the benefit of a carburetted Diesel other than getting manifold vacuum for the brake booster and/or compression braking which is usually on the exhaust? I'll go do a google search....
Last edited by revetec; 02-28-2008 at 02:51 PM.
It isn't directly the higher pressure over more of the stroke that is the problem. The problem is that because it is constant pressure combustion The flame speed becomes more of a limiting factor. During a constant volume combustion the fuel burns quickly around TDC and then the resulting hot gasses are expanded. The gasses expand much faster than the flame front speed so for petrol engines flame front speed is only an issue around TDC. With constant pressure combustion, fuel burns during most of the power stroke, meaning that the piston must travel slower than the nominal flame speed to extract work.
Relax there is no advantage at all. Remember I said it was a small model diesel engine. The engine is only roughly 1cc and it was carburetted because it is easier to make a tiny carburettor then a tiny fuel injector for direct fuel injection. The engine could not be easily throttled and when he did change the speed he used a combination of changing the CR and opening or closing the fuel needle in the carb. I just mentioned it to show that RPM has to do with piston speed and flame front speed in a diesel.Originally Posted by revetec
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
As a note: I drove my Mercedes Vito Van (diesel) on the weekend, and it revs to about 6,000rpm
Last edited by revetec; 03-02-2008 at 03:20 PM.
Selected 1st gear and gave it full throttle.
I think my tacho goes to 7,000rpm and redlines at 6,500rpm.
This is not mine, just an image I found on the web of a LHD cluster.
Mercedes Vito Van Instrument Cluster
Last edited by revetec; 03-02-2008 at 04:34 PM.
Which engine does yours have?
all the engines in the Vito peak at 3800RPM...
That means that you are somehow able to rev it 2200RPM past peak HP?
Sounds alittle unbelievable to me.
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
Maximum power is at 3,800rpm but it revs way past that. Did you look at the tacho? I'll try and get a picture of mine tonight.
Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
Engine torque is an illusion.
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