[SuperGirl_Dad]

Lets start this by posting the BS4/BS6 emission levels:



Image source Autocar

Be it BS4/BS6 some key takeaways:

1. CO emissions is higher in petrol
2. All other emissions much lower in Petrol compared to diesel

Other general things (these are typical values, but lets just agree with these to a reasonable extent):

3. Petrol FE is ~20% lower to diesel in comparable configuration/driving condition
4. Diesel is ~10% cheaper than petrol pan India.
5. Diesel has ~40% more torque in comparable configuration - few exceptions

Diesel emissions are more difficult to control and with CAFE standards kicking in, diesel will soon be extinct in next 10 years. Even MUVs are turning into petrol only models.

There has been all sort of hybrids. Lexus NX350h even has a separate electric motor at rear axle to become a AWD (Lexus NX 350h Review).

Now with this cleared; why has a combination engine of both petrol and diesel been never attempted, be it BS4 or BS6?

For Eg:
XUV500 has 2.2L engine for both petrol and diesel. In fact the petrol engine is a reworked diesel engine, as mentioned in review (Mahindra XUV500 Petrol Automatic : Official Review).



Source: T-BHP reviews

They produce similar torque and power. Both revvs to 4800 rpm. Both are turbo charged. Why not have 1st and 4 th cylinder as petrol and 2nd and 3rd as diesel or vice versa. Fuel tank is 70 liters can be divided into 30 L diesel and 40L petrol - will need two fuel gages.

Sure some might miss the high revving petrol engines, but it can result in improved FE

But the following can be achieved:
1. Petrol FE can be improved
2. Diesel emissions can be reduced.
3. average cost of refueling a 70 L combination tank will still be less than petrol only

Challenges:
- Will have to deal with two turbo engines
- Need advanced ECU to control each part of engine independently.
- emission regulations?
- Need to refuel both fuels

Twin engine cars have been attempted before but mainly performance oriented.

Maruthi is even aiming to crack biogas (whatever that is). Why cant they take their 1.5L petrol and diesel and form a combination engine instead of completely abandoning diesel?

Questions:
Why has this never been attempted?
Can this prolong diesel utilization with capability to meet strict emission regulations?
Is there any fatal flaw that is missed?

What are your thoughts on this.

[Jeroen]

There is an engine that can run on petrol and diesel. Most Jet engines are capable of running on a wide variety of fuels. But gas turbines have never been a succes in auto design, not even on trucks.

A diesel engine is a completely different design from a petrol engine. It has different compression ratio, different valve timing, different exhaust gas temperatures, in general pistons, bearings and the cilinder head of a diesel engine needs to stronger as it will have to deal with bigger forces then on a petrol engine. The fuel systems are very different too. You can’t use a petrol high pressure fuel
Pump for diesel. You will need a two tanks, two separate fuel system from the tank, low pressure pump, high pressure (injection) pump, separate fuel rails, filters etc.

Due to the diesel pistons putting more torque on the driveshaft compared to the petrol piston you would also have a hard time balancing this engine. If you can balance it at all. Also the firing sequence or rather ignition in a diesel is very different from a petrol.

Etc etc etc

So I do not see it happening anytime soon. Besides all our cars going to be powered by electricity or hydrogen.

Jeroen

[SuperGirl_Dad]

Originally Posted by Jeroen

A diesel engine is a completely different design from a petrol engine. It has different compression ratio, different valve timing, different exhaust gas temperatures, in general pistons, bearings and the cilinder head of a diesel engine needs to stronger as it will have to deal with bigger forces then on a petrol engine. The fuel systems are very different too. You can’t use a petrol high pressure fuel
Pump for diesel. You will need a two tanks, two separate fuel system from the tank, low pressure pump, high pressure (injection) pump, separate fuel rails, filters etc.

Dear Jeroen, Yes I do understand the concerns of separate fuel lines, turbo, filters, tanks and pumps.
The valve timing and other engine parameters can be controlled by using programming the ECU.
If the output torque is the same (same engine components for petrol and diesel) as mentioned in the above example of XUV500 engine, why will balancing be a problem?

[RadixLecti]

Your idea was interesting to think through.

Jeroen has already mentioned several problems with implementation.

Originally Posted by SuperGirl_Dad

If the output torque is the same (same engine components for petrol and diesel) as mentioned in the above example of XUV500 engine, why will balancing be a problem?

The peak output torques are similar, not the same, and each occurs at a different speed range (and the span of the two ranges are different). This is because the two engines, even if they have the same bore/stroke, and some common components, do not have the same torque curve.

If the two engine halves are rigidly coupled (as they are, in your thought experiment) then they are forced to run at the same speed. So with two different torque curves, one half will be doing more work than the other at almost every speed. This is what is meant by balanced/unbalanced operation - that all cylinders are doing the same/different amount(s) of work. (NB, this has nothing to do with rotational balancing of the moving parts, which may be what you are referring to).

You can either live with the unbalanced operation, which means stresses on the moving parts, or you can adjust one half of the engine up or down to match the other (by this I mean adjusting fuel supply, valve timing etc to achieve the cylinder load and pressures required). I'm sure this is possible, but it means operating one half of the engine overloaded or underloaded. In either case it will not be at the 'best compromise' point between efficiency and emissions, because you have added another factor to the compromise - the other engine half.

Plus the operating temperatures are different - this would thermally stress the block and head if they are common to the two halves.

I believe it would be cheaper and easier to make two separate smaller engines and couple them in some way which allows each engine to run independently.

[SuperGirl_Dad]

Originally Posted by RadixLecti

Your idea was interesting to think through.

Plus the operating temperatures are different - this would thermally stress the block and head if they are common to the two halves.

I believe it would be cheaper and easier to make two separate smaller engines and couple them in some way which allows each engine to run independently.

I understand there are technical challenges. Maybe a V configuration can make things better for thermal management?

A couple of known acquaintances are looking to sell well maintained diesel XUV500s in their 6th year of ownership. Both <80k run automatics in NCR. This will move even GTO to tears, who advocates for longer ownership. If they wait even 2 more years, their resale will plummet. Upgrading to Diesel BS6 is not on the cards due to DPF. So both are looking at petrol/hybrid (XUV700, Hycross) and that's a additional 10L spend. Both are not looking at EVs as it will be mainly for highway drives. Fast charging is not even cheap (A quick, unplanned solo drive to Kanyakumari in an Electric Car (MG ZS EV))and dont know what benefit hybrid brings to highway drives as well.

From another thread, the automotive industry has given this as a solution to diesel through BS6:
"-- Forced Parked Regeneration Starts (The Flights of Icarus | Our Mahindra XUV700 AX5 Diesel MT) --

On stationary mode in an open area, I cranked up the engine with the gear in neutral and with the handbrake pulled up. Then I accessed the "DPF Regeneration" section under "Drive Info" menu (after Trip A, Trip B & Clock), and pressed "OK". The mHawk engine immediately revved up to 2,500RPM by itself and stayed there continuously. The CoTek told me that it would take at least 30 minutes for the process to be complete if the soot accumulated inside the DPF (Diesel Particulate Filter) had exceeded 10 grams, and not to touch any of the A, B or C pedals. I sat inside quietly and kept observing the MID which kept showing the 2 warnings. After 7-8 minutes, the warnings disappeared from the MID and the engine revs came back to normal idling.

-- Forced Parked Regeneration Ends --

Revved it a couple more times and then drove for 40 kms on the open highway before returning home."

So automotive industry suggests revving for 10 mins and burning fuel is better to environment as there is overall less pollutants .

I'm appalled no research is made to make things easier and simpler to common man with respect to diesel emissions.

If hypothetically this engine can be made, wouldn't the overall emission be less compared to say only diesel (not comparing to petrol)?

[Jeroen]

Originally Posted by SuperGirl_Dad

Dear Jeroen, Yes I do understand the concerns of separate fuel lines, turbo, filters, tanks and pumps.
The valve timing and other engine parameters can be controlled by using programming the ECU.
If the output torque is the same (same engine components for petrol and diesel) as mentioned in the above example of XUV500 engine, why will balancing be a problem?

RadixLecti explained the challenges of balancing quite well already. In addition the diesel combustion process is very different from petrol combustion process. We think of it as more or less immediate ignition of the fuel/air mixture. In reality it is a very complex process that happens in milliseconds, but causes additional forces on the piston and piston rod and crankshaft and all the bearings.

so what you have is a crankshaft that gets very irregular forces applied to it during its rotation. Is it possible to balance such an engine sufficiently? In all honesty I don’t know. It has been forty years since I was involved in such calculation. But it is going to be a major design challenge.

The valve timing is controlled by variable valve timing gear (VVT) which by itself is controlled by the ECU. But the VVT mechanism itself is a very complex mechanical piece. I can’t even begin to imagine how that would work if you would have to combine a petrol and diesel version on one and the same engine. Also it means you will need to have separate camshaft for the diesel and the petrol cilinders! An alternative would be to have individually electronically controlled inlet and outlet valves. Those do exist, but I don’t think I have come across them on small car engines.

Dont underestimate the challenges that come from having two different cycles (diesel and petrol) running in one and the same engine block and cilinder head. Very different temperatures and pressures involved, which all need to be accounted for in the design and the manufacturing of these parts.

It all means you need to “bolt” on all kinds of additional stuff, which all reduces efficiency, makes the total engine package more heave and increases its total volume too.

Jeroen

[srini1785]

Some points that came to my mind. These are purely engineering design challenges. The efficiency part comes later.

1. Petrol engines have a compression ratio of 9:1 to 12:1, Diesel Engines have a ratio between 14:1 to 25:1. So in a 4 cylinder engine if 2 are petrol and 2 are diesel, it would be quiet a task to maintain compression ratios if all pistons are in the same crankshaft.

2. Timing : Petrol engines have spark plugs while Diesels have Fuel injectors. it would not be easy to synchronize these two in a firing sequence in a multi cylinder engine.

3. Two fuel tanks : Not that its a big task but FE of Diesels are less than a petrol so generally Diesels need bigger tank than a petrol engine of same capacity. So where do you put that extra tank?.

So considering all the above, wise people decided to make separate engines.

[SuperGirl_Dad]

Originally Posted by Jeroen

So considering all the above, wise people decided to make separate engines.

@srini1785: are you sure about the third point.

From XUV500 petrol review:
"One major downside (and the biggest for most people) will be the petrol's fuel-efficiency. Simply put, it will be a petrol guzzler; Mahindra hasn't even shared its ARAI rating . The big engine, hefty kerb weight and torque converter AT will together ensure that FE remains in the single digits. We expect it to give anywhere between 6 - 7 kmpl in the city (5 if you have a happy right foot (Mahindra XUV500 Petrol Automatic : Official Review))."

Generally FE of diesels are much higher than FE of petrol's. Hence suggested splitting 40 L for petrol and 30L for diesel in the xuv500 70L fuel tank.

Question is can this prolong diesel utilization (even in BS6 standards) more effectively?
Can this help in meeting stricter emission standards?

If hypothetically this engine can be made, wouldn't the overall emission be less compared to say only diesel (not comparing to petrol)?

[alpha1]

Originally Posted by SuperGirl_Dad

Other general things (these are typical values, but lets just agree with these to a reasonable extent):

3. Petrol FE is ~20% lower to diesel in comparable configuration/driving condition
4. Diesel is ~10% cheaper than petrol pan India.
5. Diesel has ~40% more torque in comparable configuration - few exceptions

Diesel emissions are more difficult to control and with CAFE standards kicking in, diesel will soon be extinct in next 10 years. Even MUVs are turning into petrol only models.

Challenges:
- Will have to deal with two turbo engines
- Need advanced ECU to control each part of engine independently.
- emission regulations?
- Need to refuel both fuels



Questions:
Why has this never been attempted?
Can this prolong diesel utilization with capability to meet strict emission regulations?


What are your thoughts on this.

As with everything else in life, when you do business, there has to be a business case for any new investment that one does.

Few things:
1) What customer challenge are we solving? Emissions?
Customer hardly cares as long as it meets the Govt stipulations (Euro/BS norms)
Survival of diesel engine?
Who cares, people will jump to any fuel that makes their overall cost low without hampering availability of the vehicle and fuel.

2) If we want to address the fuel efficiency, you have stated that Diesel engines are more efficient as well as Diesel fuel is lower priced, so why would customer want to move to hybrid MS-HSD engine? By the way Hybrid EV offer high fuel efficiency and lower emissions compared to any other combo.

[AROO7]

Originally Posted by SuperGirl_Dad

For Eg:
XUV500 has 2.2L engine for both petrol and diesel. In fact the petrol engine is a reworked diesel engine, as mentioned in review (Mahindra XUV500 Petrol Automatic : Official Review).


They produce similar torque and power. Both revvs to 4800 rpm. Both are turbo charged. Why not have 1st and 4 th cylinder as petrol and 2nd and 3rd as diesel or vice versa. Fuel tank is 70 liters can be divided into 30 L diesel and 40L petrol - will need two fuel gages.

Questions:
Why has this never been attempted?
Can this prolong diesel utilization with capability to meet strict emission regulations?
Is there any fatal flaw that is missed?

What are your thoughts on this.

I don't know your background (as in undergraduate degree) but a simple search of Otto cycle and Diesel cycle & petrol and diesel engine working will tell you that what you are suggesting here is defying the fundamentals. See one needs to understand petrol and diesel are two completely different fuels. Some key differences, former is lighter and latter is denser. Former has less energy per cubic volume compared to the latter (thatswhy diesel is more fuel-efficient).

Due to these and few more inherent characteristics of the fuel the engine construction and the way they are ignited is completely different. In diesel engine, air is compressed and gets heated in the process which is sufficient to ignite the sprayed diesel. So the engine block needs to be stronger hence a diesel engine is heavier. In petrol the fuel and air is pre-mixed and sprayed inside the cylinder and ignited with a spark plug so the engine block doesn't need to be that strong so they are lighter. Now if you gel these two the very first thing would be weight imbalance even if you keep other things which will go haywire aside for the moment. Good luck driving such a car.

Also members above have already pointed out several other reasons why this is not possible and since it is not possible no one ever tried to make one.

Coming to your worry about emissions, see when DPF regeneration happens the heated exhaust is used to burn the soot and not throw back the pollutants in the atmosphere. These if not for the DPF would have gone into the atmosphere (commonly seen in the form of thick black smoke when older diesel engines are revved hard). So DPF regeneration doesn't increase pollution.

With the CAFE II norms as evident from your table the petrol and that too turbocharged ones are in danger. Since they have a very high CO level, because when CO+O2 happens we get CO2 a greenhouse gas. Thatswhy all the research work is focused on petrol hybrids and EVs.

Coming to petrol hybrids, I concur you might have heard these phrase 'engine based on Atkinson cycle' (if you have time just read a bit on that). If not here is the short version, it makes very less power lower down the rev range and works like a normal engine in the higher rev range. To compensate the loss in power lower down a battery-electric setup is used. That's how you save fuel without sacrificing on power and also the reason why petrol strong hybrids give the best fuel economy when driven in city.

Also one needs to understand by burning diesel or petrol we are just releasing the stored carbon inside the earth in the atmosphere. So both the fuels are detrimental to the environment and will have to go one day in future. Thus making petrol hybrids just a stop gap measures and EVs & Hydrogen-powered vehicles the future as BHPian Jeroen stated. Leading to a point where now there is no incentive (for the industry) to work on internal combustion engine or even think on a petrol+diesel engine or any thing of such kind as BHPian alpha1 mentioned.

[athalaga]

Originally Posted by SuperGirl_Dad

Diesel emissions are more difficult to control and with CAFE standards kicking in, diesel will soon be extinct in next 10 years. Even MUVs are turning into petrol only models.

CAFE norms are fuel efficiency, Diesel scores over Petrol.
RDE introduced at BS VI phase 2, have same norms for both engines.
Euro 7 is already launched in Europe.

Diesel hardware is already introduced and complete with BS 6 (DPF, SCR), tweaking is on test methods and calibration. Diesel can go as far as Petrol can go to meet same regulations.

Let us not spread mis information here.

[Tgo]

Originally Posted by SuperGirl_Dad

If hypothetically this engine can be made, wouldn't the overall emission be less compared to say only diesel (not comparing to petrol)?

Lets go this way…

Emissions from burning a teaspoon of petrol and diesel are different because of the different physical and chemical composition of the fuel. Petrol is a lighter hydrocarbon burns cleaner whereas diesel is heavier and burns less efficiently.
Fact 1: lighter hydrocarbons burn cleaner

Similarly, since lighter hydrocarbons have less energy density. That is why diesel engines are more fuel efficient, because they use less of it to do the same work.
Fact 2: lighter hydrocarbons have less energy density

Now you take that teaspoon and pour it inside a cylinder (it has to be atomized and should have the correct air to fuel mix) and burn it to create an explosion. That is your engine basically. The emissions in this case keeping the same cylinder dimensions would depend on how the fuel atomization happens differently for diesel and petrol, the flame travel speed, position of piston at ignition point, temp of air, compression ratho, temp of the piston/cylinder etc…

Fact 3: BS norms stipulate how much an engine should pollute (max limit). Manufacturers develope the engine to meet or be under that limit.

Fact 4: BS norms are supplemented by fuel quality specifications too. That is why we have BS6 fuels which have evolved over the years and are a standard product whether it is made by IOCL or Reliance.

Fact 5: Due to different fuel property, managing emissions in petrol and diesel engines is done differently. O2 sensors and feedback loop in petrol, aided by catalytic converters for petrols and DPF and urea injection in diesels to name a few.

Fact 6: You manage emissions based on the fuel you are burning.

What you are asking is, mixing petrol with diesel will give you good of both. Clean burn from petrol and fuel efficiency of diesel. Unfortunately that’s not how physical properties of a mixture ever work. The resulting mixture is a new beast altogether has to be studied separately, an engine to run it has to be developed from scratch leaving behind 100+ years of development of the internal combustion engine, at the perceived fag end of fossil fuels. Can be done if there is a will. Like how Porsche is developing a synthetic fuel and an engine to run it. Whether it will give the best of both fuels…?? Thats an unknown.

[SuperGirl_Dad]

Originally Posted by athalaga

CAFE norms are fuel efficiency, Diesel scores over Petrol.
RDE introduced at BS VI phase 2, have same norms for both engines.
Euro 7 is already launched in Europe.

Diesel hardware is already introduced and complete with BS 6 (DPF, SCR), tweaking is on test methods and calibration. Diesel can go as far as Petrol can go to meet same regulations.

Let us not spread mis information here.

Dear athalaga; Thank you for taking time to reply, meant BS6 phase 2 norms. Higher fraction of diesel cars getting axed in Indian market owing to BS6 phase 2 norms has been posted in our own forum. Please see post #32 in link. Lets stick to Indian context and lets not go off topic with this please.

Originally Posted by Tgo

Lets go this way…

Emissions from burning a teaspoon of petrol and diesel are different because of the different physical and chemical composition of the fuel. Petrol is a lighter hydrocarbon burns cleaner whereas diesel is heavier and burns less efficiently.

What you are asking is, mixing petrol with diesel will give you good of both. Clean burn from petrol and fuel efficiency of diesel. Unfortunately that’s not how physical properties of a mixture ever work. The resulting mixture is a new beast altogether has to be studied separately, an engine to run it has to be developed from scratch leaving behind 100+ years of development of the internal combustion engine, at the perceived fag end of fossil fuels. Can be done if there is a will. Like how Porsche is developing a synthetic fuel and an engine to run it. Whether it will give the best of both fuels…?? Thats an unknown.

Dear Tgo, thank you for your detailed reply. interesting read! I think our thought process are at a tangent.

"What you are asking is, mixing petrol with diesel will give you good of both...The resulting mixture is a new beast altogether " - I DID NOT ask for mixing both fuels .

I simply asked in case of XUV500 2.2 L engine with same engine block and same internal components shared between petrol and diesel models, can the 1, 4 cylinders be petrol and 2,3 cylinders be diesel or vice versa. Engine ancillaries including tanks will be two separate entities. The petrol cylinders will burn ONLY petrol and Diesel cylinders will burn ONLY diesel.

Can this help in meeting more stringent emission standards for diesel models?

As our esteemed members pointed out there are several technical challenges and I do agree. Should be an interesting case study for any automotive Masters thesis project, atleast if any simulated study can be performed to gauge the emission levels.

Dear mods: If you feel the thread has run its course you can lock it.

[Kosfactor]

If you have a diesel car with a perfectly functioning DPF and SCR - urea injection system etc it will be as clean if not cleaner than a petrol engine without all those things.

So no need to mess with it, they are wonderful engines.

Diesels really got the bad name because of one company - VW.

[xsrahul]

Actually there is latest research on Dual Fuel engines with alternate combustion properties - like Gasoline and Diesel.

This engine christened ‘RCCI’ short for Reactivity Controlled Compression Ignition, was developed at the Engine Research Center at the University of Wisconsin–Madison.

Interestingly it uses the both fuels at different times in the combustion cycle to produce a high thermal efficient (60%) which is higher than Gasoline or Diesel alone.