OHC vs. OHV Diesel Engines: Car Comparison

Hi there,

I possess both design principles for driving camshafts.

OHC with timing belt: VW 1.9 TDI with VT-ESP and 2.0L gasoline engine (all using the 827 engine block).
OHV engine with spur gear set: Volvo B30 (3.0L 6-cylinder gasoline engine).

Whenever I look at a Volvo B30 engine, I always wonder what advantage an overhead camshaft actually provides in a diesel engine.
- No extreme valve timing requirements for the diesel engine (no gimmicks like adjustable camshafts are needed).
- No crossflow principle is necessary in the cylinder head to achieve better efficiency.
- no high RPMs are required (the B30 can easily handle 6000 rpm, and it's not a short-stroke engine).

What makes Overhead Camshaft (OHC) engines appealing in diesel applications?

In an overhead valve (OHV) engine, the camshaft is located low, and a spur gear set provides simple, inexpensive, and maintenance-free drive with a long service life.
It would also be quite feasible to drive the ESP using a bevel gear set (see, for example, the Toyota 2H or 12-HT engines).
And the planetary gear set located in the engine oil circuit experiences no stress due to external contamination (which makes off-road enthusiasts happy ).

Explain to me why the world of (car diesel engines) is the way it is .

Regards,
Jens.

OHC engines are more compact in design compared to OHV engines. Additionally, they have fewer components that can potentially fail.

...
In an overhead valve (OHV) engine, the camshaft is located low, and a spur gear set provides simple, inexpensive, and maintenance-free drive with a long service life.
...

With many beautiful moving masses that really shake and vibrate, and which additionally require unnecessary energy. This means that even more energy must be expended for compensating waves, further reducing efficiency.
At the same time, the weight of the entire engine increases, which puts a barrier in the way of lightweight cars with low fuel consumption. Perhaps Ulf could say something about how to reduce weight .


By the way, I even know of bumper motors that can easily spin at over 10,000 RPM... but I don't want those vibrations from those kinds of motors. Most of them shake themselves apart.

TDI-GTI-4-Motion wrote:

OHC engines are more compact in design compared to OHV engines.

I doubt that.

Volvo B30 vs. Mercedes M110.
VW 2.1L WBX vs. Subaru 2.0L.

In both cases, the overhead valve (OHV) engines are noticeably "more compact"; an OHC engine cannot have such a slim cylinder head as an OHV engine. However, the increased space required in the engine block due to the camshaft and pushrods located below is hardly noticeable.

Take a look at a stripped-down WBX; it's hard to believe that such a small engine can push a 2.5-ton truck and still have a 2.1-liter displacement.
Anyone who occasionally looks into the engine bay of a T3, once with a 2.1L WBX and once with a 1.6L TD, will immediately understand what I mean .

goettmann wrote:

With many beautiful moving masses that really shake and jiggle.

Experience or theory?

A Volvo B30 is not at all a "shaking engine"; it's also an inline-six cylinder engine, which means it doesn't have primary or secondary unbalanced forces.
But actually, the vibration-inducing effect of an overhead valve (OHV) timing system should be particularly noticeable in a 6-cylinder inline engine. I guess I'm just not noticing it .
However, you can definitely hear the valve train; I'll admit that. The pushrods and rocker arms aren't particularly quiet.
But is that really that important for diesel engines?

Jens 16syncro wrote:

OHC engines are more compact in design compared to OHV engines. TDI-GTI-4-Motion wrote: I doubt that. Sure, there are exceptions. But try designing a block for an OHV and an OHC engine to be as slim as possible. You can't tell me that the one for the OHV is thinner. Somewhere, the camshaft has to go.

And from a height, it barely makes any difference.

TDI-GTI-4-Motion wrote:

And from a height, it almost doesn't take anything away.

And why do the cylinder heads of the two example OHC engines mentioned have a much lower profile than those of their corresponding SOHC engine counterparts?

Using side-mounted rocker arms that directly act on the valve stems allows for a much lower design. An overhead camshaft almost inherently requires more height because it sits on top .
Okay, that's obviously nonsense. Even OHC engines can have a side camshaft or use similar designs to save height. Nevertheless, a classic OHC cylinder head with direct actuation of the valves via tappets is significantly taller than a classic OHV cylinder head.

I haven't read any compelling arguments in favor of OHC yet, at least none that I can relate to the example engines.

"How about:"
OHC is much cheaper to produce in large quantities than OHV. From the manufacturer's perspective, this would already be a very significant argument.
Is that really the case?

...
However, you can definitely hear the valve train; I'll admit that. The pushrods and rocker arms aren't particularly quiet.


Every sound that you can hear requires (unnecessarily) energy to be created...

VW 2.1L WBX vs. Subaru 2.0L

Which specific 2.0L Subaru are you referring to?
They are available in all three configurations: OHV, CIH (a hybrid between OHV and OHC), SOHC, and DOHC. The most compact of these is the DOHC .

Jens 16syncro wrote:

... I haven't read any compelling arguments in favor of OHC yet, at least none that I can relate to the example engines. ...

If you assume that an engine is just a static block of metal, then I don't have much to add. Unfortunately, that's not the case.
Due to temperature, the block expands significantly in height, which results in a large variation in valve clearance in OHV engines. By shortening the valve actuation mechanism (as in OHC engines), you eliminate all that hassle.

Quote:

... even OHC engines can have a side-mounted camshaft...

No, those are CIH engines .

Bertil wrote:

Due to temperature, the block expands significantly in height, which results in a greatly fluctuating valve clearance in OHV engines. By shortening the valve actuation mechanism (as in OHC), you avoid all this trouble.

Hi,
I am familiar with that argument, but it doesn't convince me either.
It seems that at WBX, it is understood that the valves only rattle when the hydraulic lifters are defective .

The "modern" camshaft drive system in VW OHC engines, which uses a bevel gear set, suffers from the same problem because the backlash between the teeth changes due to thermal expansion, requiring complex compensation measures. The adjustment in OHV valve drives is much simpler to implement, thanks to hydraulic lifters.
Given the low RPM range of a diesel engine, it seems like that should be easily manageable.

I know that the discussion is detached from reality, but there must be some concrete and verifiable facts to explain why overhead valve (OHV) engines are becoming obsolete. I agree with this for gasoline engines, as OHV would severely limit the freedom in designing the cylinder head for optimal airflow. However, it doesn't pose a problem for diesel engines.

Okay, at Volkswagen, the OHC (Overhead Camshaft) design was implemented because the underlying engine block, the 827, was originally designed for an OHC configuration. However, this is a production-related historical reason and has nothing to do with insurmountable disadvantages of OHV (Overhead Valve) engines.

Camshaft driven by gears?
Have you ever heard of a T5 with a 2.5-liter engine? It's really strange, and whether it will last longer than a regularly replaced timing belt is something we won't know for sure until after 10 years.

Jens 16syncro wrote:

I am familiar with that argument, but it doesn't convince me either.

It's quite simple: more components lead to higher costs, which creates the potential for savings of millions.
This will be the only real reason why it's no longer being used.

RedR32 wrote:

Timing chain of the camshaft? Have you ever heard of a 2.5-liter T5 engine?

It's really weird, and whether it will last longer than a regularly replaced timing belt is something you only know after 10 years. }

"Yes, I drive one of those. What's weird about it? Where does it say that a diesel engine has to be quiet? And between you and me, gears last longer than timing belts; otherwise, you would have to overhaul your transmission every x thousand kilometers. Take a look at how engines in trucks and other large motors are controlled..."

And the T5 is basically a truck. Personally, I only hear the loud front-wheel drive noise when the vehicle is cold. When it's warm, it just sounds like what only a five-cylinder engine can sound like, and some people actually like that. We are among them.

It's quite simple: more components lead to higher costs, which creates the potential for savings of millions.
This will be the only true reason why it is no longer being used.

It's not quite that simple... But it's definitely close.

But let's ask a different question:
Why OHV? What would be the advantages?

When it comes to engine height, OHV designs are hard to beat. However, this design increases the engine's width, which can make packaging difficult in transversely mounted engines and compact cars. The space required for the entire OHV mechanism is significantly larger compared to OHC designs. Ultimately, the valve actuation system runs four times from the engine block to the cylinder head.

More moving mass in an overhead valve (OHV) engine increases fuel consumption. Especially today, when we want fuel-efficient engines, this is a definite 'no-go.'

It's important to consider that with an OHC engine, the entire block needs to be designed differently. This means that the overall direction of the engine development must be determined early on.
Converting a 2V OHV engine into a 4V engine is hardly possible without completely rebuilding the block and the cylinder head. In other words, the effort involved is extremely high. With an OHC engine, on the other hand, it is much easier. It only requires a new cylinder head and minor modifications to the camshaft drive. Therefore, the engine block remains largely unchanged. With an OHV engine, you would need two completely different blocks.

With the wide range of engine variants available today (2-valve, 4-valve, etc.), it's particularly important to have a platform that can cover as broad a product portfolio as possible without major modifications.
The OHV can hardly offer that.
Since diesel engines are now commonly equipped with 4-valve cylinder heads, a two-track approach to design and production is not practical.



While the overhead valve (OHV) engine may be suitable for specific applications, this design is no longer ideal for cost-effective mass production.

goettmann wrote:

But let's ask a different question: Why OHV? What would be the advantages?

I dream of a diesel engine that has a maintenance-free drive system for the camshaft, oil pump, and fuel injection pump, and which is 100% resistant to external contamination. By this, I mean that the engine should be able to be submerged in mud up to about half its height without any expected consequential damage.

Quote:

When it comes to the building height,

Yes, even a low height is beneficial for my dream engine.

Quote:

For that, the engine needs to be wider.

Well, the overall width of the engine block mainly determines the arrangement of the auxiliary components. The space required for the camshaft located next to the crankshaft seems less important in that regard.

Quote:

The space required for the complete overhead valve (OHV) mechanism is significantly greater compared to that of an overhead camshaft (OHC) system.

Well, a B30 engine doesn't really look that huge. Compared to an M110 DOHC, it's actually quite a slim unit.

Quote:

The valve actuation system is ultimately routed from the block to the cylinder head four times.

Camshaft on hydraulic lifters, then pushrods, then rocker arms (if the camshaft is properly positioned in the engine block). The hydraulic lifters and rocker arms need to be guided; in my case, there are only two guide locations.

Quote:

Given the current range of engine variants (2-valve, 4-valve, etc.), it is particularly important to have a platform that can cover as wide a product range as possible without major modifications. The OHV can hardly offer that.

Okay, here's the translation:

"Ok, that's the core."

Quote:

Certainly, the overhead valve (OHV) engine has its justification for specific applications, but for cost-effective mass production of a wide range of variants, this design is obsolete.

Yes, unfortunately...

By the way, I know of an engine that comes pretty close to my ideal design:
Toyota 12H-T
4.0L inline six-cylinder engine, overhead valve (OHV).
Mon - Tue - Wed, Turbo, Inline ESP.

Unfortunately, it's more of a truck engine, so it's much too large and heavy.

Building your "dream engine" is not a problem, but paying for it.