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Old 1st July 2017, 09:10   #1
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Default Explained: Why Diesel engines need a throttle body & throttle plate

Why do Diesel engines need throttle body and a throttle plate?
Short answer: Diesels do not need throttle body and throttle plate for its efficient operation.

Why?
Because Diesel engine is throttled by fuel not air like in a Petrol engine. Give more fuel in a Diesel engine and it turns faster with more torque, give less fuel it slows down. Maintaining stoichiometric ratio is not as important as in a Petrol engine to keep a Diesel engine running.

In a Petrol engine to make the engine turn faster the throttle plate is opened allowing more air into the engine and ECU/Carburetor feeds more fuel to maintain stoichiometric ratio for the given air supply.

Then why does some Diesel engines come equipped with a throttle body?
Yes some modern engines have throttle bodies and some do not. The primary reason for its presence is emissions control and Exhaust Gas Recirculation (EGR). The ECU by means of electrical motor or solenoid controlled vacuum line can control the actuation of the butterfly valve in the throttle body to restrict the amount of fresh air (from turbo charger) into the intake manifold under low engine load situations. This helps create a pressure differential and thus helps the EGR system to feed more exhaust into the intake manifold.

This is what Delphi has to say about their Diesel engine throttle bodies.
Explained: Why Diesel engines need a throttle body & throttle plate-untitled.jpg
(Src: http://www.delphi.com/manufacturers/...ottle-controls)

Main reason for having a throttle body on a Diesel is EGR and it also has a role to play during engine shutdown. When engine is turned off the valve closes and cuts air into the cylinders this helps to achieve a relatively smoother shut down. Throttle body works as an Anti-Shudder Valve (ASV).

Another reason people say a throttle body could be of some use is during an engine runaway situation by cutting off the air supply. But this would work only if the throttle body is in closed position for as long as the key is OFF. In the Crysta the throttle body is spring loaded to be always open not closed so may not be effective in this type of a situation.
Explained: Why Diesel engines need a throttle body & throttle plate-bosch.jpg

Examples of Diesel engines with and without throttle body?
With: Toyota GD and KD engines, VAG 1.6, 1.5 and 2.0 CR TDI engines, Fiat 1.6 MJD engine.

Without: Fiat 1.3MJD and Ford/PSA 1.4 and1.5 TDCI engines.
These are what I can recall now for with and without from the engines sold in India.

What does one achieve by removing the throttle plate?
Diesel engines like air and more air. Better the airflow into and out of the engine better the performance and lower the Exhaust Gas Temperature (EGT). By removing the throttle plate I hope to gain two different but related advantages - 1) I remove the tiny restriction in the intake manifold. 2) I prevent the ECU from creating a pressure differential in the intake manifold to aid more exhaust gas in through EGR.

1GD-FTV Throttle Body
It may not be as evident in the photos below, but when the throttle is wide open the throttle plate is not perpendicular. The throttle plate is slightly angled down at its rear (where it meets manifold). This means there would be a slight restriction in air flow in comparison to if the the throttle plate was completely perpendicular when wide open.
Explained: Why Diesel engines need a throttle body & throttle plate-20170616_16125001.jpeg
Explained: Why Diesel engines need a throttle body & throttle plate-20170616_16130601.jpeg

1GD-FTV Intake Manifold
See the bump on the roof of the manifold? Thats where EGR connects to the intake manifold (not seen in the photo).
Explained: Why Diesel engines need a throttle body & throttle plate-20170616_16132202.jpeg

Throttle Plate Delete
The screw holding the throttle plate to the spindle was locked as expected. It does not turn and easily strips the head if force is used and your bit is good! So the only option was to drill the screws out. It took me some time as the spring loaded throttle plate would sit fully open in its natural resting position preventing the access to the screws. So I had to with one hand hold the throttle plate closed and with the other hand manage the hand drill and drill the screw out. The throttle spring was pretty stiff in this one, much more stiffer and stronger than one would expect in a Petrol engine.
Explained: Why Diesel engines need a throttle body & throttle plate-20170616_17154401.jpeg
Explained: Why Diesel engines need a throttle body & throttle plate-20170616_17460501.jpeg

Better leave the spindle there, so as to feed the ECU about its position. Not a pleasant sight loosing that much area to the spindle cross section. HaHaHa!

This is probably the first documented case of throttle plate removal in a GDFTV engine. I have not come across another one as on date.

Verdict
There is improvement in off idle throttle response, now it just got a bit more urgent while accelerating. The difference while noticeable is not huge.

The compressor surge noise which was audible earlier is now almost gone. Letting go off the throttle pedal after accelerating hard ECU decides to partially close the throttle to aid the EGR. ECU does this whenever leg is off the pedal. When the throttle valve close boost has no place to go. This causes surge/stall. With no throttle body to close the boost is consumed by the engine. This is the reason why some highly modified Diesels include a diverter valve/blow off valve in their build, to prevent stall/surge. Anyway I am not going that route.

Swirl Flap
I am just mentioning it here since few have questioned the effectiveness of the throttle plate delete in the presence of swirl flaps which are semi closed during low load low throttle situations.Intake runner has two ports, one has a swirl flap and the other one is open always. So even if the swirl flap is semi closed the boost can enter the cylinder through the other port. So having swirl plates does not nullify the throttle valve plate removal. At high RPM and high load swirl flaps are fully open.
--

PS: This modification is Post Unichip installation.

Last edited by Sankar : 1st July 2017 at 09:11.
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Old 3rd July 2017, 13:54   #2
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Awesome post, Sankar - thanks for sharing! Moving it out to a new thread.
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Old 3rd July 2017, 16:09   #3
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Very interesting post.

Quote:
It may not be as evident in the photos below, but when the throttle is wide open the throttle plate is not perpendicular. The throttle plate is slightly angled down at its rear (where it meets manifold). This means there would be a slight restriction in air flow in comparison to if the the throttle plate was completely perpendicular when wide open.
Well, it would be a very slight restriction in air flow. But there might be another reason as well. It might be done on purpose to ensure the airflow swirls around after leaving the throttle body.

Just thought I would share the throttle body of my Jaguar XJR. 4.0L, Super Charged V8.

Notice the two cooling water pipes. This throttle body is hooked up to the engine cooling system. All it does is prevent “icing”. Some performance die-hards will disconnect these line to try and get lower air temperature into the engine. In practice it doesn’t seem to make a whole lot of difference.

There is also a suggestion that it would prevent carbon deposit building up on the plate and the bore.

When I just bought my car in 2009 I had some problems with the engine management system going into ‘restricted mode’. An OBD read out pointed towards the throttle body and I found there was a technical bulletin from Jaguar highlighting problems with the throttle body. Notably the potentio meter on the side of the throttle body was giving problems. It measures/transmits the position of the plate to the ECU.

A new throttle body cost around $2.500 and with 4-5 hours installation time is easily a $ 3.000,-- repair! I only paid $10-12.000 for the car in the first place, so I nearly fainted. For the first few years of this car Jaguar had carried out the replacement at no cost as part of warrantee. Luckily, I managed to convince them to extend that courtesy a few years later to me as well!

I always thought it utterly ridiculous to replace a $2.500,-- part, if really only a $10,-- (i.e. the potentiometer) was broken.

In the aftermarket and through various Jaguar forums/clubs you can obtain the potentiometer and instruction on how to install/calibrate these days.

From above:

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From the side:

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Note the black “thingy”. This is the potentiometer

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Jeroen
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Old 3rd July 2017, 16:53   #4
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

I don't want to go off topic on my own thread about Diesel throttle body. I have done the Coolant bypass in my petrol Swift (now sold) and noticed that it in practice it did make a difference in my car. You are right heated throttle body is to prevent icing. Later versions of the same car did not have the coolant circulating in the throttle body.

Quote:
Originally Posted by Jeroen View Post
Notice the two cooling water pipes. This throttle body is hooked up to the engine cooling system. All it does is prevent “icing”. Some performance die-hards will disconnect these line to try and get lower air temperature into the engine. In practice it doesn’t seem to make a whole lot of difference.
Throttle body coolant bypass can be discussed here:
http://www.team-bhp.com/forum/diy-do...-bypass-3.html (DIY : Throttle Body - Coolant bypass)
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Old 3rd July 2017, 18:11   #5
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

S-Cross 1.6 has throttle body. But it is not present in 1.3 MJD.

Explained: Why Diesel engines need a throttle body & throttle plate-img_20170703_180017.jpg

Thanks a ton for explaining the EGR and throttle body connection in diesel engines. I want to share something here. One of my friends with 1.6 S-Cross had got the remap from Quantum UK. EGR was deleted in the map. But he complained about loss of power at low end / crawling speed in respective gears. Quantum UK gave him a new map and the issue was rectified. I asked him to log and the EGR was ON in this map.

This is just my analysis :

When EGR was soft deleted in the map, throttle body was still creating the vacuum to feed in more exhaust gas. So, the fresh air was compromised and also the exhaust gas was not going in here as the EGR was disabled. This resulted in power loss at low end. I might be wrong here, but just thought of sharing my view

I want to do the same throttle body mods like you have done in my S-Cross and soft delete the EGR with remap. This should give a clear picture.

Last edited by Dr.Naren : 3rd July 2017 at 18:13.
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Old 3rd July 2017, 19:37   #6
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

How does it affect the engine braking, post delete? Running down the hill the diesel engines have the tendency to free run to high rpm, even in low gears. Ideally the plate should offer some resistance to the airflow, helping the engine braking. Just a thought.
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Old 3rd July 2017, 19:39   #7
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Also I would think less excess O2 => less NOx.

Thank god for this post. Many times I thought of posting about diesels and stoichiometric ratios, but held back not wanting to deal with googlers who would gleefully point out diesels with throttle flaps.

Anyone remember the Matador setup?

Re: engine runaway, all diesels need a overspeed preventer/ governor.
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Old 3rd July 2017, 20:52   #8
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Dr.Naren View Post
Thanks a ton for explaining the EGR and throttle body connection in diesel engines. I want to share something here. One of my friends with 1.6 S-Cross had got the remap from Quantum UK. EGR was deleted in the map. But he complained about loss of power at low end / crawling speed in respective gears. Quantum UK gave him a new map and the issue was rectified. I asked him to log and the EGR was ON in this map.

This is just my analysis :

When EGR was soft deleted in the map, throttle body was still creating the vacuum to feed in more exhaust gas. So, the fresh air was compromised and also the exhaust gas was not going in here as the EGR was disabled. This resulted in power loss at low end. I might be wrong here, but just thought of sharing my view

I want to do the same throttle body mods like you have done in my S-Cross and soft delete the EGR with remap. This should give a clear picture.
Your analysis could be true. But recirculated exhaust is supposed to be inert and works by reducing the displacement available for fresh air. So EGR should not aid in combustion, but its absence may affect cylinder filling if the throttle valve is closed at the same time. Ideal way to find out is for the map writer to check the calibration, if it is possible.

Years ago some have experimented with EGR disable (physical no remap) in the Hyundai Elantra CRDI (1st model) and had reported a loss of torque. I don't know if that 2.0 CRDI had a throttle valve. Does it?

In my ex 1.3MJD there was no perceptible loss of torque with EGR solenoid disconnected. I ran with EGR solenoid disconnected till i got a remap. No throttle valve in that engine.

Quote:
Originally Posted by bejoy View Post
How does it affect the engine braking, post delete? Running down the hill the diesel engines have the tendency to free run to high rpm, even in low gears. Ideally the plate should offer some resistance to the airflow, helping the engine braking. Just a thought.
Crysta is automatic so not an ideal candidate to evaluate engine braking, but I find no difference in how it slows down post delete. I had a VW 1.5 GT TDI and also a Vento 1.6 TDI which i bought used and both are MT so ideal for evaluating engine braking; these cars are equipped with a throttle valve like the Crysta but there is no effective engine braking when comparing these to a petrol engine.

Throttle valve will only help in engine braking if its programmed to keep valve closed for as long as foot is off the pedal. Unlike petrols in which valve position is closed by default due to spring tension, the one in 1GD-FTV is open by default due to spring tension. If valve is closed and EGR is open it still gets exhaust gas for cylinder filling. There is no strong vacuum like a petrol motor. Its the vacuum which helps with engine braking.

Quote:
Originally Posted by Sutripta View Post
Re: engine runaway, all diesels need a overspeed preventer/ governor.
Did you mean the valve acting as a speed governor or as a device to prevent runaway?

To kill a runaway Diesel the sir supply needs to be cut off. Throttle valve closes when engine is being turned off. But this is momentary and it springs back to open when power is cut off when key is off in this motor. So not sure if it will help with a runaway, the valve needs to remain closed for some time to kill runaway diesel. This is why I did not mention it in the first post. There are reports on the internet of engine runaway in ASV/throttle valve equipped TDI engines.
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Old 3rd July 2017, 21:41   #9
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Dr.Naren View Post
I want to do the same throttle body mods like you have done in my S-Cross and soft delete the EGR with remap. This should give a clear picture.
Hmmm... think about it. It is bit difficult to reverse it. Actually reversing depends on how it is removed. A professional with the right equipment and (patience ) will be able to grind off the other side of the screw (which is sort of flattened like a rivet to prevent unintentional loosening) and unscrew the throttle plate. I knew I won't be reversing it thus did this irreversibly. Those who go the full monty do the complete removal of throttle valve assembly apart from other flow enhancing mods.

In the long run I want to take off the complete EGR system off and blank both sides and loop the coolant.

Quote:
Originally Posted by Sutripta View Post
^^^
The overspeed preventer is built into the fueling system.

Regards
Sutripta
Yes thats what i thought.

Last edited by Sankar : 3rd July 2017 at 21:53.
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Old 3rd July 2017, 21:43   #10
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

^^^
The overspeed preventer is built into the fueling system.

Regards
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Old 3rd July 2017, 22:39   #11
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Sutripta View Post
^^^
The overspeed preventer is built into the fueling system.
When you say fueling system could you eloborate a bit what you mean/how it works.

I have little experience working on car diesels. All my company cars were diesels (because it's more economic) but I never work on my company/lease cars.

So my diesel experiences is with marine and stationary (power) diesel. In most cases overspeed preventer is built into the governor, be it mechanically or electronically. I.e. what we would call a ECU or similar in a car. Ultemately it does control the fuel system of course. But I'm puzzled on how the fuel system would recognize an overspeed condition. Maybe semantics?

Maybe I'm missing something? Happens a lot these days! I'm alomost sixty!

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Old 4th July 2017, 13:23   #12
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Jeroen View Post
When you say fueling system could you eloborate a bit what you mean/how it works.
Jeroen
As far as I know in modern ECU controlled diesel engines, the speed limitation is done by a governor (software) which in turn results in fuel quantity. The same applies to both Engine overspeed limitation and vehicle overspeed limitation, only the input parameter (error signal) to the governor is different.


The other use of a throttle is to create vacuum inside the brake booster. The Brake Booster basically requires vacuum in order to aid braking and this vacuum has to be regenerated after every application of the brake pedal. Some vehicles have a dedicated electrical vacuum pump others use the throttle valve, the throttle valve is closed when he engine is running and hence air is drawn out of the vacuum chamber inside the brake booster through a one way valve.

Last edited by Whiplash7 : 4th July 2017 at 13:38.
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Old 4th July 2017, 14:05   #13
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Fantastic piece of information. I was not aware till now that some diesel engines do have a throttle plate.

Can you please explain what happens if you let go of the accelerator suddenly after hard acceleration IF the throttle plate is still there? In your case, as you explained the absence of the throttle plate now will dissipate the boost into the engine. But what if the throttle plate were there? Will the boost get dissipated slowly or is there a BOV type of mechanism to let go of the extra pressure while decelerating?
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Old 4th July 2017, 15:21   #14
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Whiplash7 View Post
The other use of a throttle is to create vacuum inside the brake booster. The Brake Booster basically requires vacuum in order to aid braking and this vacuum has to be regenerated after every application of the brake pedal. Some vehicles have a dedicated electrical vacuum pump others use the throttle valve, the throttle valve is closed when he engine is running and hence air is drawn out of the vacuum chamber inside the brake booster through a one way valve.
All diesel engines have a vacuum pump to generate vacuum, most of them are engine driven. Throttle plate is not used to generate vacuum in a Diesel. Electric vacuum pumps are used by some cars, even petrol cars.

Quote:
Originally Posted by audioholic View Post
Can you please explain what happens if you let go of the accelerator suddenly after hard acceleration IF the throttle plate is still there? In your case, as you explained the absence of the throttle plate now will dissipate the boost into the engine. But what if the throttle plate were there? Will the boost get dissipated slowly or is there a BOV type of mechanism to let go of the extra pressure while decelerating?
On some Diesel engines with a sharp ear (and poor sound insulation) you can hear the turbo surge when letting go of the throttle. This would be more prominent if the engine is running a tune with higher than stock boost*. Even if there is a throttle plate it does not close completely like a petrol so boost is consumed by engine.

I used to hear that noise in the Crysta and now with the plate gone there is almost none. Preventing surge/stall helps with response.

Some people chose to install a BOV/diverter in their diesel builds. But since there is no vacuum in the manifold to open the diverter they trigger it electronically.
--

* In the VW TDI I have not heard the surge though the car was remapped. Maybe its not there or the insulation is too good (which it is in VW).

Last edited by Sankar : 4th July 2017 at 15:29.
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Old 4th July 2017, 15:28   #15
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Default Re: Explained: Why Diesel engines need a throttle body & throttle plate

Quote:
Originally Posted by Sutripta View Post

Anyone remember the Matador setup?
IIRC, Matador have a throttle plate and the accelerator cable have a 'T' at the end one part goes to throttle plate and another one goes to FIP. The same setup is used in Nissan SD engine with an inline FIP.
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