Interesting typo in HJ's test of the new Colt - could be a useful (if unlikely) entry among the touring cars!
"1,493cc 12 valve twin cam 3 cylinder diesel: 70kW (944bhp) at 4,000 rpm; 210Nm (155 lb ft) torque at 1,800rpm."
It reminded me that I had also seen the 'twin cam' label on an Isuzu 4WD recently, and I wondered why anyone would go to the trouble of doing that to a diesel? This isn't one of my usual diatribes about diesels, BTW, it's just physics - you simply don't need low reciprocating mass valvegear on a low revving engine, so I can only assume it's done for show, or perhaps because the heads are shared with a petrol model. Does anyone know?
The Colt review mentioned the new Renault Modus, which is now being advertised on the box, and quite neat it looks. I imagine the Colt will remain the better buy, but I did recently see a Renault Trafic Van with Nissan badges, so who knows what you're getting these days?
Over and out...
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Recently had to change a cambelt on a Bedford Movano van, (Renault engined I think). That was a twin cam and like yourself I wondered why. I would think that the average Movano owner would be more interested in simplicity, longevity and reliability, and a belt driven twin cam engine can't add much there.
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Have you seen the Peter Simple column in todays D.Torygraph? Long awaited promotion for you :-)
Happy Quaffing Phil I .
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"Peter Simple column"
Thank you, Phil. Nice to know someone's paying attention! :-)
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Hi,
Two possibilities spring to mind:
1) The four valve per cylinder layout allows a good, central, easily cooled position for the injector.
2) The two canted inlet valves can give the inlet air a sort of barrel roll in-cylinder motion, which may be useful in mixing air and fuel, while keeping the fuel off the cylinder wall, during the injection(s)
I suppose if I were considering the design of such an engine, I might think about making the inlet ports to two patterns, i.e., so each cylinder has one of each pattern. One optimised to give good swirl at low engine speeds, which would become restrictive to airflow at higher engine speeds, and the other port optimised for higher engine speeds. This would give a very flexible engine, which would possibly be well suited to commercial vehicle use.
number_cruncher
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N_C wouldnt that make for a slighty eratic engine?
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Was mech1
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Hi Steve,
Erratic in which way?
number_cruncher
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It wouldn't be erratic if the variation of airflow to each port was suitably controlled, as in the case of some twin turbo (large and small) set-ups.
Cheers, SS
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I'm just wondering why any modern motoring enthusiast would want to issue a diatribe against diesels? It's a bit like being against slimline flat-screen computer monitors or compact discs!
Cheers, SS
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"against slimline flat-screen computer monitors or compact discs"
Hmm - at the least they don't smell, rattle or make roundabouts lethal for motorcyclists! As an enthusiast, I like engines with a wide power band and a crisp throttle response.
As it happens, I prefer a good CRT for colour rendition and contrast. Another advantage is that they can function optimally at more than one resolution. I'd like a flat screen for aesthetic reasons, but I shall be sticking with CRT's for the moment. Ooh, and I quite like vinyl, too... :-)
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"The four valve per cylinder layout.."
Any why would you need that? Lots of valves serve the same purpose as DOHC's - they reduce mass and allow higher revs. IIRC, you get better swirl and combustion at low revs with two valves per cylinder.
I think it's marketing...
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Mr Jagworth,
The reason for multi valve engines isn't mass - it is airflow. The more valves you have in a given cylinder diameter, the larger the area for air to flow through. Having the inclined valves in a pent roof chamber allows slightly more valve area still.
Having overhead cams as opposed to side mounted cams is to do with reducing valve train mass. Reducing valve train mass allows you to either i) allow the engine to rev more before valve bounce or ii) reduce the valve spring stiffness and/or preload, so as to reduce engine friction and wear.
Over the years, there have been many schemes tried in order to get good swirl in diesel engines. One of the odder of these was on AEC engines where part of the back of the inlet valve was masked. The valve ran in a keyed guide to stop it rotating.
Another good way to get good mixing is to use a piston with a toroidal central cvity, which will provide lots of squish as the piston reaches TDC. This type of piston may be difficult to use in conjunction with a multi valve head.
The problem is that in order to get swirl, which you need to get good fuel/air mixing, you tend to restrict the airflow in some way. Although diesels do tend to run very weak, you still want to get all the air you can into the cylinder.
number_cruncher
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Yes, but smaller valves have less inertia, so they can reciprocate faster, which is only necessary if the engine has to rev quickly. At lower revs, the airflow isn't a big issue, because there's more time available, so I return to my original question, why DOHC's (and multi-valves) on a diesel?
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Mr Jagworth,... may I call you J?
why DOHC and multi valves on a diesel?
Multi valves on diesels because you want to get as much air in as possible - preferably moving and swirling in a good way to help efficient combustion.
Some (relatively historic) large industrial engines have four valves per cylinder to get good breathing. Because they are so slow running, they have side cams and pushrods.
How would you actuate inclined multiple valves in a pent roof combustion chamber? (inclined and pent roof to give maximum valve opening area)
number_cruncher
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\"may I call you J?\"
Of course. Not many people are that polite!
I hear what you say about the breathing, but you\'ve already agreed that swirl (and hence good combustion) requires a certain amount of restriction, and if the engine is running relatively slowly, you\'ve got plenty of time to get the air in (especially if it\'s being assisted by a turbo).
IIRC, multi-valve petrol engines are not always as tractable at low revs as their 2-valve cousins, and I even have some recollection of an engine design that only used all four valves at higher revs, for that reason.
If you have multiple valves and a pent roof, then I agree that two cams are a good idea - I\'m just not convinced of their relevance to engines that operate below, say, 5000 rpm.
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Hi J,
At low engine speeds, yes, the benefits are minimal, but why limit diesels to low engine speeds?
The upper limit on useful diesel engine speed is, for modern engines and materials, being set by combustion rates rather than inertial stresses.
The reason why some multi valve engines may be untractable at low speeds is probably more to do with their valve timing rather than the fact they are multi valve engines. i.e., they have the equivalent of a racier cam profile.
number_cruncher
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I wasnt certain what you meant. ie 2 valve 4 valve per cylinder. Which was why I said. Your point on a 4 valve would need controlled air intake to both valves. Sofa Spud also mentioned. though he didnt say what type. apart from turbo.
>>The upper limit on useful diesel engine speed
not sure if this now applies. as some Diesels are now used to race with. so as Diesels are competing with petrol.I think Diesel winning the race. catching up.I do not think comments that were made before reflect on modern engines.?
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Was mech1
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Hi Steve,
Please forgive me if I misinterpret your post - I'm not sure I follow it entirely [your style uses significantly fewer words compared with my verbosity :-)].
I am not sure I understand the need for controlling the air input into diesel engines - they tend to run very weak anyway. Even at full load, there is typically 20% excess air. I suppose as long as you can measure how much air goes in, you can make sure that you don't overfuel and cause excessive particulate emissions. If you don't measure the air that goes in, you may need to use a 'safer' fuel mapping to avoid soot, etc.
Racing engines usually do not need to demonstrate either good fuel consumption, or more saliently, acceptable emissions. (Wouldn't it be good it formula 1 and the like did have to pass euro V or whatever the current rules are? Then, the money spent on F1 might have some benefit for ordinary motorists!)
number_cruncher
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