Technical Analysis: WRC Cars 2017

[Mirek]

Yes but 8500 is quite extreme and I somehow doubt that such high rpm are useful while being problematic for reliability.

[J_n_z]

I have no special engine knowledge just little more than basic thermodynamics so... Reliability from which reason?
Simply moving to fast or thermal stress on engine parts, oil...?

[Mirek]

I am also no engine engineer but all S2000 teams had issues with broken valve springs (partially due to the rules - too heavy valves), also connecting rods are stressed very much by high piston speed (due to piston acceleration).

[dimviii]

1,6 lit ,36mm restrictor and 8500rpm will be usefull only at big straights when you want as much possible km/h.

[Sulland]

I am also no engine engineer but all S2000 teams had issues with broken valve springs (partially due to the rules - too heavy valves), also connecting rods are stressed very much by high piston speed (due to piston acceleration).

Are the teams allowed to use lighter valves on the WRC 2017 generation, to avoid valveflow?

[Mirek]

I'm sorry, my English is probably not good enough, but I don't know what You mean by valveflow.

Anyway I don't know the exact WRC rules.

AFAIK the problem with S2000 valves was that they had to be made of steel, i.e. they were heavy. With such high engine speed the valve springs had to cope with very high valve inertia. For me it is an example of a wrong idea how to make things cheaper.

Dim: That's exactly what I though - if the higher top speed as the only clear benefit is worth the effort to make the engine reliable at so high rpm. Maybe they simply set the limiter to lower value than what is allowed by the rules.

[janvanvurpa]

I am also no engine engineer but all S2000 teams had issues with broken valve springs (partially due to the rules - too heavy valves), also connecting rods are stressed very much by high piston speed (due to piston acceleration).

Mirek my dear friend,

Maybe you're not engine engineer but you don't have to be to think...These are 2000cc motors maybe 81mm bore..that means there is only room for 2 x what size valve? Whatever it is, not very big. And if its not very big then even if it had a "big" fat 7mm valve stem like oh, I dunno, the "safe past 9500 rpm ancient old 1970s Ford Cosworth BDG" it wouldn't weigh "that much".

Now I will bet the valve stems are 5,5mm or maximum 6mm , but probably 5,5mm---that means, even in steel, it weight less than old BDA valve...

PLENTY little street car Hondas live just fine at or above 8500 rpm..

Valve failures in all liklihood came from design choice, making them super lightweight not too heavy..

High piston speed...there you can look at the general trend toward longer stroke---because smaller bores..

Smaller bores is driven by traverse engine mounting to some degree (Honda mentioned that last century, first in their inlne 4 bikes --its narrower, then in their CVCC traverse engines "shorter engine fits better when sideways than big bore" they said.) and burn efficiency (takes less time to burn 41-42mm from center (in 4v pent roof head with the sparkie plug right in the middle) than it takes to burn across 90 or 94mm or 96mm with a plug waaaaay over on the side) old school stuff like I build.. This smaller bore thing is a big part of why these modern motors can have scary high compression AND on turbo motors, high boost.

Design choices..8500 isn't remarkably high...I've read the rules carefully and I don't recall seeing any mention of a minimum valve weight---which means that a designer can go crazy and make the valve ultra light---I say they just 'got too greedy'.

[Mirek]

Jan, it might sound weird but I am sure that the problems with broken valve springs on S2000 cars were real and not that uncommon.

On the other hand I think that only few times Fabia (the car with the longest stroke - 92 mm) suffered from broken conrod and that was due to a faulty batch of them. Otherwise even with brutal overrewing usually something else broke.

The long stroke actually doesn't apply to 1.6 WRC engines built from scratch (usually bore/stroke around 83/73). The only 1.6 WRC engine with long stroke was Mini due to the fact that it was the only one based on production unit (PSA/BMW EP6CDT).

[dimviii]

a reason for broken valves can be also from ''soft'' valve springs.
They try to use soft springs for less losses .How soft you can go is a compromise between less losses and precision valve drive at high rpms.
Soft valve springs (softer than ideal I would say) drives to valve float at high rpms,and that's can be dangerous for engine durability.
Of course the reason can be also and a bad quality batch of valve springs.

[J_n_z]

I may be wrong, but in term of absolute numbers, wrc engines don't seem too high tech to me. F1 Honda '88 engine with same 2,5 bar boost reved to more than 12000... I know F1 is not relly, but it was 30y ago.

I can imagine that it is tricky to achieve optimal effect with limited amount of air (optimal combustion), but it should not be problem to make it mechanicly reliable...