Why Don't Diesel Engines Have an Exhaust Bypass?

Why not ask your Disesel Engine Supplier this reason, is this not a better idea?....than asking every Tom, Dick, Harry & CR4s

Because there is oks like myself who could benefit from better answers!

This question has resulted in two very good answers from two very knowledgeable folk - 17 GA votes combined. Seems asking here was not such a bad idea.

Seems like a reasonable question to ask here, and I'm sure someone will be glad to try and help you find an answer. Unfortunately that person is not me.

Good luck.

Agree, a reasonable engineering question.

rizwan, when you say "exhaust bypass" do you mean some means of bypassing the turbo charger? If so then the answer is that it is not necessary to do so. The "boost" from the turbocharger is pretty much self regulating by design of the compressor and expander curves and it is load dependant and does not need to be "lowered".

Hi,

I think you might need to clarify a little. These lots of differnet exhaust paths on modern diesels. I.e. Exhaust Gas Recirculation, exhaust bypass around turbos, etc.

DB

GA from me.

Thats exactly my take on the way things work.

Dennis,

if you were a women....I would have kissed you for your answer althou you are wrong on some parts!

Thanks mate!

Dieselphil is correct about diesel combustion. The use of common rail piezo activated injectors found on the newest European diesels significantly lowers the "knock" and the manufacturers hope the low NVH will increase acceptance of diesel engines in passenger cars in the US market despite the fact that diesel has historically been more expensive here than gasoline. (The reverse is true in Europe.) However, it is important to note that direct injection gasoline passenger engines, which will appear on the US market for the first time this year, also benefit from turbo charging. The direct injection produces a stratified charge in the compustion chamber and allows the engine to operate with a very lean fuel mixture. However, I don't know whether those engines can operate without a wastegate on the turbocharger since unlike large truck and marine diesels they operate over a wide speed range. I understand those turbos will have variable geometry.

Dennis Waller

Lets keep it simple, shall we? First, the terms (assumptions and facts).

1) Gas in this query is natural gas fueled engine, not gasoline (although concept is the same since no liquids on earth burn), or shall we say "Spark Ignited" engines. This generally means fuel enters the combustion chamber, is compressed and waiting for ignition, backwards from a diesel (compression ignition)

2) The bypass, as Phil points out, is a "waste gate" and some older models have shims on the spring loaded bypass valve that resists opening pressure to adjust for altitude.

The answer:

3) Diesel engine are not subject to fuel pre-detonation so the more air we can cram in the better we are to get in more O2, and then we can jam in more fuel and get more power and get the rating higher and kick the competition in the pants (on paper)

4) Spark ignited engines do not like hot intake air (from the turbo compressor) and will pre-detonate when the piston also adds compression on the up stroke, so, we want to slip some exhaust out and slow down the turbo compressor wheel speed. Remember, spark ignited engines have the fuel and air mixed already and can detonate when the temperature of that mixture exceeds the auto-ignition point of that fuel. Diesels do not have the fuel mixed during compression, only red hot air that is waiting for the precise moment of fuel injection and flame front building as more fuel is vaporised into the flame ball. Ta-dah.

5) At high altitude with little air density, and therefore fewer molecules bouncing against each other under compression by the turbo and piston (less heat), we increase the turbo speed (put shims in the spring of the waste gate to make it harder to open the bypass circuit hence increase compressor wheel speed and cram in the same amount of air as we might at sea level (sort-of). But at low altitude there would be too much air (heat) so we fit the turbo with a waste gate on spark ignited engines to control air fuel ration and detonation risks of any spark ignited engine.

For the pedantics out there, please don't condemn this simple Simon answer as I am writing it for the general audience without need to throw my diploma on the table and get into stoichiometric equations, 'N' gasses and peak pressures.

Cheers

A couple of points to ponder:-

1) Good turbos feed through an inter cooler to get the charge temperature down and therefore get more O2 in....(lower temp means more atoms!)

2) I have driven turbo diesels up to several thousand feet and you do not notice any power loss, partly/mostly due to the fact that as air gets thinner, the turbo turns faster and crams more of the thinner air in - end result you get the same amount of O2 in the engine in the end......why would you need to change the waste gate setting - especially as most Diesel Turbos do not have a waste gate......

3) Some modern petrol engines also use direct injection, they prove to be even more economical and detonation should be a thing of the past then.......surely?

This is not criticism, just a discussion....

Have a great day.....

Hi Andy

Almost all industrial diesels have aftercoolers these days... can't think of one without it. Cars do also, and some inner coolers (same concept it is just mounted 'inside' (inner) the block instead of a hang on device just after the turbo (aftercooler).

You don't typically have waste gates on small diesels. Small to me is less than 3 MW. Only spark ignited engines have waste gates (except as mentioned below). Only when fuel and air enter the combustion chamber do you need traditional waste gates on smaller engines.

For diesels, small turbos can spin up to 130,000 RPM and in fact at high altitudes they can blow up rather nicely (seen it in Andes of Colombia)

Large engines that I work with such as Wartsila have waste gates on diesels but that is to limit the RPM and centrifugal forces, not air ratio.

I guess we are saying the same thing.

Cheers

JO!!!

Injecting gas or petrol:

Ya buddy! I am playing with large engines now (8 MW) that inject natural gas at 350 barg. Pilot ignition is with liquid fuel such as heavy oil or crude oil, which ignites under compression ignition (diesel cycle) and just as the flame front builds we inject the natural gas into the flame. No pre-detonation since no fuel mix is compressed with the piston, no spark plug needed, and the gas can be 'heavy' (full of constituents that one cannot use in a spark ignited engine since those heavy constituents pre-detonate).

Cool 'eh ?

There is an exhaust bypass (waste gate) in many diesel engines. When you have an engine like f.example in a car where you are constantly accelerating and decelerating then it is good to have a waste gate because when you accelerate from low speed then the turbine has not reach its speed immediately and is not pumping air into the engine and is actually blocking the air to the engine. Then you need this bypass so you can have enough air into the engine until the turbine has reached more speed, this way you can pick up speed faster and you save fuel, it does not matter if it is gas, diesel or anything else.

In many applications like f. example some gen sets and pumps and main engines in vessels, the engine is running most of the time on the same speed and load and then the bypass has nothing to do and is just a wast of money.

I hope this answers your question

That does not say anything logical......sorry, have another go.....

A wastegate is a way of controlling the pressure produced by a large turbo so that it supplies the correct amount of air needed at all times AND DOES NOT "over" charge the motor with air/fuel at other times.....it evens the turbo charging out to what the engine can handle at all times. But needs to be physically larger to handle the low engine speed situations.

Today a turbo would be used with variable geometry.....and no wastegate....

Many years ago a colleague had a Turbo Subaru, the waste gate control was broken, so they fixed the wastegate closed. His car was suddenly a real rocket (till the repair was made!!). He "accidently" overtook the engineer that had diagnosed the problem in the first place, on the autobahn, going far far faster than he was actually allowed by the motor design!!! A shorter engine life was probably the end result....

You misunderstand Andy

I never mention "over charge"

At idle speed the turbine is doing almost nothing, the higher the speed of the engine, the higher pressure is produced by the turbine.

You never have to bypass the tubine to lower the air pressure, when the engine is accelerating it needs more air than the turbine can deliver this is why you have to bypass.

In some turbines this problem is solved with an electrical motor to prevent the turbine running to slowly .

Study before you disagree.

Then you appear to understand very little about the actions of a "Waste gate" on earlier Turbo car installations.....

For a fixed blade turbo to work effectively at low and high speeds, it has to be made to "blow" at low engine revs with little exhaust pressure.

If no waste gate was installed, this would imply too high a pressure at higher engine revs.

This is why the waste gate, usually boost pressure activated, but can also be controlled electronically, opens to "waste" the exhaust pressure into the exhaust pipe, without passing through the Turbo itself - Wasting energy if you like.....

May I recommend the following to be read to "update" your knowledge on this subject:-

http://en.wikipedia.org/wiki/Waste_gate

This an interesting and accurate document on waste gates and how they work.....if you don't want to believe me, perhaps Wikipedia may convince you!!!

"You never have to bypass the tubine to lower the air pressure, when the engine is accelerating it needs more air than the turbine can deliver this is why you have to bypass."

Huh? How does a waste gate accomplish that? Waste gates merely allow exhaust pressure to bypass the turbo impellers, in effect slowing the turbo down to reduce boost. How a waste gate, plumbed into the exhaust, allows more intake air to flow around the turbo is a little beyond us here.

Good post.

Nobody else understood what he meant either......even he appears not to as well.....