I learned recently that many older and some newer diesels don't have throttle plates to control airflow. I was also quite shocked to learn about a condition called diesel engine runaway. Apparently a diesel engine can run out of control from oil pooling in the manifold.

It seems that not having throttle plates on older diesel engines has it's risks. Why have newer diesel engines begun to adopt a throttle? What are the ultimate benefits of incorporating a throttle plate into the intake system?

I can see benefits regarding preventing diesel runaway conditions. Risk for diesel runaway can be mitigated with intake design to prevent the oil pooling condition in the intake manifold, so why integrate a throttle plate? Would a throttle plate assist in greater level of efficiency for fuel consumption?

  • I'm wondering ... since diesel engines are direct injected ... how does diesel pool at intake manifold? Diesel shouldn't even be getting there. Maybe someone can clue me on this? Commented Dec 29, 2015 at 17:16
  • I believe this is pre-direct injection??? @FredWilson has some knowledge regarding this. He's been my diesel mentor. Commented Dec 29, 2015 at 17:18
  • Even indirect injected diesel engines don't put fuel in the intake manifold. Commented Dec 29, 2015 at 17:23
  • 1
    If a Diesel engine limited air flow using a throttle, then you wouldn't get the temperatures created by compression required to ignite the fuel, would you?
    – HandyHowie
    Commented Dec 29, 2015 at 17:35
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    @Paulster2 Indeed you did. I guess I'm still iffy on the runaway thing and throttle plates. This is an emotional issue for me now :-| Commented Dec 29, 2015 at 17:35

3 Answers 3


tl dr: By adding a throttle plate, it creates the vacuum needed to draw in gasses from an EGR valve.

Since diesel engines are designed to run lean, they don't need throttle plates to run. They utilize the amount of diesel fuel needed to keep the engine running and to provide the work needed to do the job required of them. One of the inherent issues with running lean is with a lean burn, you also run hotter. If the burn is hotter than ~1700 degF, you start to form Nitrogen Oxides (NOx), which is the key ingredient in acid rain and will tear up people's lungs (therefor is nasty stuff).

One of the ways to deter the formation of NOx during the combustion cycle is to introduce an Exhaust Gas Recirculation (EGR) process. The spent exhaust gasses provide a means by which to control the burn process thus reducing the heat in during the combustion process. This has been used for many years in gasoline engines. Unfortunately with diesel engines, since there isn't a high level of vacuum in the intake, it won't readily draw the EGR gasses into itself and therefore is self defeating. By adding a throttle plate, it creates the vacuum needed to do the draw.

  • Is that the EGR valve you are talking about?
    – HandyHowie
    Commented Dec 29, 2015 at 18:19
  • @HandyHowie _ I believe that's what I was getting at ... Commented Dec 29, 2015 at 18:32
  • I have been reading up on this, it seems that this extra throttle (if you really can call it that) is just there to increase suction and therefore flow of inert gasses from the EGR valve. It does not lower the total volume of gas inhaled by the engine as in a gasoline engine throttle. If the gas volume was throttled, the compression process would not create the temperatures required to ignite the fuel.
    – HandyHowie
    Commented Dec 29, 2015 at 21:16
  • @HandyHowie - Yes, that is what I believe I said in my answer (last line). Let me update my answer just a tad. Commented Dec 29, 2015 at 21:22
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    Another thing to mention is that unlike a gasoline engine that has maximum EGR flow under power, a diesel has maximum EGR flow at idle. This requires the "throttle" to only be closed at idle to improve EGR flow and open otherwise.
    – vini_i
    Commented Dec 30, 2015 at 0:23

Anti Shudder Valve

Not a direct answer, but one engine component that many people confuse with a throttle plate on a diesel engine is called an "anti shudder valve". This looks exactly like a throttle body, but it only has two positions - fully open and fully closed.

On an engine equipped with the device, the valve is completely closed when the key is turned off, completely starving the engine of air. This causes the engine to shut down quickly and smoothly without any "shuddering" that might happen if the engine continued to take in and compress air as the rotating mass comes to a rest.


Over here . .

Why do heavy vehicles almost always use diesel engines?

Someone wrote; "I can get umpteen Nm of torque from a motorcycle engine and a big gear ratio, yet they do not use them in heavy vehicles. So, torque alone is not the answer".

In response; yes, I can understand how you could come to that conclusion, as I love/ride bikes and also have one that is turbocharged that possesses lots of torque.

That said, simply put, torque production with high durability and low friction (the latter two mostly coming from low engine speeds and the use of heavy duty powertrain design approaches) is really the main reason diesel engines are used. Extrapolating this and also your earlier response about motorcycles; if you look at Motorcycle engines of comparable size/cylinders to small car engines of similar capacity, you will see that the car manufacturers often still decide to adopt major design changes to their powertrain - rather than just using the same engine design approach.

So clearly there are different considerations and these come down to how the torque is manifested and delivered by various engine configurations and manufacturers.

These design changes are due to the fact that the car (and particularly truck) engine must produce more torque, and - if possible - more if it, down lower in the rev range; in order to provide the required thrust for all the (varying) weight the car itself always possesses and can carry.

Motorcycles, on the other hand do not have quite such a large potential for varying weight (as cars), and as such their engines don't need to be straddled with these same design limitations/specifications; hence their emphasis on high rotational speeds, light weight, high volumetric efficiencies, and KW - rather than (specifically) torque.

Additionally, motorcycles also are, by/large, (sold on them being) performance oriented machines, and in any event (particularly for the ones less than 1000cc) that means that they usually must rotate their crankshafts at reasonably high speeds in order to produce meaningful torque and power. This means (amongst other considerations) motorcycle engine designs - unlike small engine passenger cars - don't have to compromise high crankshaft speeds for low down torque; as most car engines designed - as above stated - do - as those car engines will simply not rev comparably high even though the same capacity engine could otherwise (in a motorcycle ) easily be designed to. So, we have an engine design trend for vehicles (that are designed to carry varying weight) that reads like this; more *constant/high value torque over a greater rev range, preferably starting as low as possible within the rev range, and with efficiency, reliability, and economy if possible.

Motorcycle engines fail at the first *specification and as such they can never do this for the reasons above, others, and also because Torque is a product not just of the combustion process and its resulting forces - but also because it is a product of the engines rotating/reciprocal weight; inertial torque. And, motorcycles (particularly their engines' rotating components) are usually quite light - not in the least to achieve the high revs they need to produce.

Therefore a motorcycle engine/design not only fails to produce the meaningful values of (inertial and composite) torque where it is needed in order to perform the tasks of heavy vehicles - but the torque it does produce is largely combustion-force dependent, and as such (even with modern gearbox design approaches) is still too susceptible to changes in weight and vehicle ascension/gradient for the tasks required.

This design limitation and problem (associated with applying motorcycle engines to heavy vehicles) largely and most obviously manifests itself as a bore, stroke, reciprocal weight, and torque bandwidth issue.

Try riding a motorcycle around the city - especially if it is hilly - that has a passenger and/or (particularly a) motorcycle trailer attached to it, and you will see not only how impractical it is to grab 4K/rpm - 5K/rpm every time you want to take off even on a really powerful motorcycle - but also you will see how long your clutch lasts and ceases to be non-smelly.

Yet, (at best/least) the same weight displacement considerations are precisely what cars must accommodate all the time and reliably; not to mention trucks. That all takes us back to my earlier comments about heavy vehicles, diesel engines, and torque; as they produce high torque values quite well, at low engine speeds, over a wide rev range, and they do it reasonably reliably too. Aside from heat, noise, and exhaust; engines only ever produce Torque and Horsepower, and the latter is a function of the former.

Reliable and cost effective torque is the name of the game, and that's why diesels where invented and it's primarily why they're mostly used in heavy vehicles today.



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