Ecotec To Go Turbo With New 3 & 4-Cylinder Engines

[dwightlooi]

The twin turbo 4 would be a question for Dwight.

Well... two turbos definitely cost more and brings with them a host of additional plumbing. The question is what advantageous do they bring?

First let's get the myths out of the way:-

• Twin Turbos are not more efficient than a single larger turbo -- in fact, they are LESS EFFICIENT as larger turbines and compressors aerodynamically superior
  
• Twin turbos (in parallel) are not more responsive than a single large turbo -- V6es use two for the convenience of not having to route exhaust to one
  

There are two advantageous to using two turbos in a 4-cylinder. Both applies ONLY to SEQUENTIAL setups...

(1) Sequential, asymmetric, turbines offer improved low end response. Basically, you have a smaller turbo which spins up sooner and faster provide boost at the lowest rung of the engine rpm range. This turbo's exhaust and waste gated bypass flow feed a second larger turbo which is able to cope with the engine's higher rpm breathing. The most advanced single turbos of today can manage a torque plateau of about ~3500 rpm at a boost level of about 1 bar (14.7 psi). This gets narrower if you run higher boost. With twin sequential turbines you can extend this by about 500 rpm. In otherwords, with a single turbo you may be able to have an engine which makes maximum torque at 2000~5500 rpm. With a sequential setup you can extend this down to 1500~5500 rpm or up to 2000~6000 rpm.

(2) Sequential compressors allow higher pressure ratios to be reached. A single stage compressor becomes very inefficient above a pressure ratio of about 2.5:1. That is, they start making more heat than compress air when asked to deliver more than about 22 psi of boost (about 37 psi of absolute pressure on an atmospheric input of 14.7 psi). By the time you reach a pressure ratio about 2.75~3.0:1 (26~29 psi of boost) more boost actually makes less power just a lot of hot air. This is why jet engines have many axial compressor stages and helicopter turbines usually have more than one centrifugal stage. With two sequential compressors you can efficiently reach 30~40 psi of boost. More if you insert an intercooler between the two stages! Realistically though, this advantage is quite irrelevant to road cars running on pump gas given that the static compression will have to be so low (~3:1) that the engine wouldn't run right if at all off boost And, if it did, would be quite lousy on thermal efficiency and fuel economy.

So... really it comes down to the ability to create one of those engines capable of 1200 rpm torque peaks or one with a modest torque peak of say 3000 rpm but a plateau stretching to 7000 rpm hence making pretty impressive power. The question is whether that added 500 rpm extension of the torque plateau is worth all the addition complexity and cost. Remember, the sequential setup is NOT more efficient or more responsive than a single setup when the latter is within its already pretty wide optimal operating range. Also, the same broadening of the torque plateau and/or increase in power output can also be achieved by running about 0.2 bar (3 psi) less boost, using a slightly larger displacement and running slightly higher static compression. In fact, the latter probably yields slightly better mpg numbers due to improved off boost thermal efficiency from the higher static compression.

[David]

The twin turbo 4 would be a question for Dwight.

Well... two turbos definitely cost more and brings with them a host of additional plumbing. The question is what advantageous do they bring?

First let's get the myths out of the way:-

• Twin Turbos are not more efficient than a single larger turbo -- in fact, they are LESS EFFICIENT as larger turbines and compressors aerodynamically superior
  
• Twin turbos (in parallel) are not more responsive than a single large turbo -- V6es use two for the convenience of not having to route exhaust to one
  

There are two advantageous to using two turbos in a 4-cylinder. Both applies ONLY to SEQUENTIAL setups...

(1) Sequential, asymmetric, turbines offer improved low end response. Basically, you have a smaller turbo which spins up sooner and faster provide boost at the lowest rung of the engine rpm range. This turbo's exhaust and waste gated bypass flow feed a second larger turbo which is able to cope with the engine's higher rpm breathing. The most advanced single turbos of today can manage a torque plateau of about ~3500 rpm at a boost level of about 1 bar (14.7 psi). This gets narrower if you run higher boost. With twin sequential turbines you can extend this by about 500 rpm. In otherwords, with a single turbo you may be able to have an engine which makes maximum torque at 2000~5500 rpm. With a sequential setup you can extend this down to 1500~5500 rpm or up to 2000~6000 rpm.

(2) Sequential compressors allow higher pressure ratios to be reached. A single stage compressor becomes very inefficient above a pressure ratio of about 2.5:1. That is, they start making more heat than compress air when asked to deliver more than about 22 psi of boost (about 37 psi of absolute pressure on an atmospheric input of 14.7 psi). By the time you reach a pressure ratio about 2.75~3.0:1 (26~29 psi of boost) more boost actually makes less power just a lot of hot air. This is why jet engines have many axial compressor stages and helicopter turbines usually have more than one centrifugal stage. With two sequential compressors you can efficiently reach 30~40 psi of boost. More if you insert an intercooler between the two stages! Realistically though, this advantage is quite irrelevant to road cars running on pump gas given that the static compression will have to be so low (~3:1) that the engine wouldn't run right if at all off boost And, if it did, would be quite lousy on thermal efficiency and fuel economy.

So... really it comes down to the ability to create one of those engines capable of 1200 rpm torque peaks or one with a modest torque peak of say 3000 rpm but a plateau stretching to 7000 rpm hence making pretty impressive power. The question is whether that added 500 rpm extension of the torque plateau is worth all the addition complexity and cost. Remember, the sequential setup is NOT more efficient or more responsive than a single setup when the latter is within its already pretty wide optimal operating range. Also, the same broadening of the torque plateau and/or increase in power output can also be achieved by running about 0.2 bar (3 psi) less boost, using a slightly larger displacement and running slightly higher static compression. In fact, the latter probably yields slightly better mpg numbers due to improved off boost thermal efficiency from the higher static compression.

You Rock!!!

Thank you for the Great explanation, so for most applications it would be best from cost, MPG and overall efficiency to stay with a single turbo and maybe consider a dual scroll turbocharger would be a step up from the single.

So if I am understanding your explanation, using dual turbo chargers really is a performance issue off the line only. How about if one were to consider dual – dual scroll turbochargers for performance with the proper intercooler for chilling to control thermals?

Could one take an Eco 4 banger and create a monster with acceptable performance?

The thoughts of what this could do for a small or medium size performance auto could be awesome I would think.

[hyperv6]

The twin turbo 4 would be a question for Dwight.

Well... two turbos definitely cost more and brings with them a host of additional plumbing. The question is what advantageous do they bring?

First let's get the myths out of the way:-

• Twin Turbos are not more efficient than a single larger turbo -- in fact, they are LESS EFFICIENT as larger turbines and compressors aerodynamically superior
  
• Twin turbos (in parallel) are not more responsive than a single large turbo -- V6es use two for the convenience of not having to route exhaust to one
  

There are two advantageous to using two turbos in a 4-cylinder. Both applies ONLY to SEQUENTIAL setups...

(1) Sequential, asymmetric, turbines offer improved low end response. Basically, you have a smaller turbo which spins up sooner and faster provide boost at the lowest rung of the engine rpm range. This turbo's exhaust and waste gated bypass flow feed a second larger turbo which is able to cope with the engine's higher rpm breathing. The most advanced single turbos of today can manage a torque plateau of about ~3500 rpm at a boost level of about 1 bar (14.7 psi). This gets narrower if you run higher boost. With twin sequential turbines you can extend this by about 500 rpm. In otherwords, with a single turbo you may be able to have an engine which makes maximum torque at 2000~5500 rpm. With a sequential setup you can extend this down to 1500~5500 rpm or up to 2000~6000 rpm.

(2) Sequential compressors allow higher pressure ratios to be reached. A single stage compressor becomes very inefficient above a pressure ratio of about 2.5:1. That is, they start making more heat than compress air when asked to deliver more than about 22 psi of boost (about 37 psi of absolute pressure on an atmospheric input of 14.7 psi). By the time you reach a pressure ratio about 2.75~3.0:1 (26~29 psi of boost) more boost actually makes less power just a lot of hot air. This is why jet engines have many axial compressor stages and helicopter turbines usually have more than one centrifugal stage. With two sequential compressors you can efficiently reach 30~40 psi of boost. More if you insert an intercooler between the two stages! Realistically though, this advantage is quite irrelevant to road cars running on pump gas given that the static compression will have to be so low (~3:1) that the engine wouldn't run right if at all off boost And, if it did, would be quite lousy on thermal efficiency and fuel economy.

So... really it comes down to the ability to create one of those engines capable of 1200 rpm torque peaks or one with a modest torque peak of say 3000 rpm but a plateau stretching to 7000 rpm hence making pretty impressive power. The question is whether that added 500 rpm extension of the torque plateau is worth all the addition complexity and cost. Remember, the sequential setup is NOT more efficient or more responsive than a single setup when the latter is within its already pretty wide optimal operating range. Also, the same broadening of the torque plateau and/or increase in power output can also be achieved by running about 0.2 bar (3 psi) less boost, using a slightly larger displacement and running slightly higher static compression. In fact, the latter probably yields slightly better mpg numbers due to improved off boost thermal efficiency from the higher static compression.

You Rock!!!

Thank you for the Great explanation, so for most applications it would be best from cost, MPG and overall efficiency to stay with a single turbo and maybe consider a dual scroll turbocharger would be a step up from the single.

So if I am understanding your explanation, using dual turbo chargers really is a performance issue off the line only. How about if one were to consider dual – dual scroll turbochargers for performance with the proper intercooler for chilling to control thermals?

Could one take an Eco 4 banger and create a monster with acceptable performance?

The thoughts of what this could do for a small or medium size performance auto could be awesome I would think.

For building a Eco here is all you need to know. This book tells how amazing this engine is and the part in it. It is a must read to really understand how good these little engines really are.

http://www.summitracing.com/parts/NAL-88958728/

[SAmadei]

Of course, all this 3 and 4 cyl forecasting will be for naught if someone figures out a way to make a synthetic, clean burning gasoline substitute from sunlight. ;-) (Assuming, of course, that Big Oil doesn't already have this secret and owns all the patents to it)

Dwight, I was curious if your expertise extends to the chemistry of gasoline. I know that gasoline is a mix of organic hydrocarbons and additives, but I've questioned for years is what compound actually makes gasoline into gasoline.

I know that Benzene, Toluene, Xylene can be removed from gas and its still gas. But what chemicals cannot be removed? Can your car run on only Butane? Or Isooctane?

Obviously, Ethanol is not quite a synthetic, gasoline substitute, as its water attracting properties and the idea we need like 10 more earths to grow all the raw materials to produce it.

Reason I bring this up is that I don't foresee a future where everyone's needs are met by 4 cyls... and I'd rather burn LPG, CNG or E85 than be straddled with a 4 cyl.

[dwightlooi]

You Rock!!!

Thank you for the Great explanation, so for most applications it would be best from cost, MPG and overall efficiency to stay with a single turbo and maybe consider a dual scroll turbocharger would be a step up from the single.

So if I am understanding your explanation, using dual turbo chargers really is a performance issue off the line only. How about if one were to consider dual – dual scroll turbochargers for performance with the proper intercooler for chilling to control thermals?

Could one take an Eco 4 banger and create a monster with acceptable performance?

The thoughts of what this could do for a small or medium size performance auto could be awesome I would think.

You will never use two dual scroll turbos on a 4-cylinder engine. It doesn't make sense from a dual-parallel turbine setup, and it doesn't make sense for the downstream turbine in a dual-sequential setup. A dual scroll turbine housing introduces a divider and hence flow impedance lowing efficiency of the turbine itself. The ONLY purpose of a dualscroll turbo is to alleviate the feed-back-pulse effect which 4-cylinder engines exhibit.

In a 4-cylinder engine, you invariably have two pistons at bottom dead center when another two are at top dead center. This means that when one cylinder (@ BDC) is opening the exhaust valve(s), another (@ TDC) is finishing its exhaust stroke. The cylinder finishing its exhaust stroke is also starting its intake stroke, therefore it tends to have both its exhaust and intake valves open at the same time. This is called the "overlap" period. For proper and complete aspiration, engines will have at least some overlap -- in the case of high strung naturally aspirated engines actually quite a lot of it! With a turbine plugging the exhaust, what happens is that the "path of lowest resistance" for the high pressure gases exiting the cylinder at the beginning of its exhaust stroke becomes the exhaust ports of the cylinder in overlap instead of the turbine! Hence, without a twin scroll turbo, exhaust is forced to back flow from the exhaust ports into the cylinder in overlap and often back up the intake port into the intake! This robs efficiency, makes the engine stumble at lower rpms and causes emissions issues. The traditional remedy is to run the minimum amount of overlap on turbocharged 4-potters. This minimizes the back flow problem, but it also makes the engine breath poorly -- especially when cruising off boost. This hurts fuel efficiency, may increase emissions and usually reduces the amount of exhaust energy initially available when the driver opens the throttle and thereby hurting responsiveness of the engine.

The twinscroll turbine housing's purpose is solely to take care of this back flow problem by keeping the exhaust flow of cylinders 1 & 4 separate from that of 2 & 3. Keeping the exhaust separate prevents backflow. The twin scroll turbine housing itself reduces the flow capacity, increases flow impedance and actually hurt the efficiency of the turbocharger itself. The back flow problem also a problem unique to 4-cylinder engines, but not inline-3 or V6es. A three cylinder engine DOES NOT have pistons at BDC and TDC at the same instance. This is why 3-potters are easier and more efficient to turbocharge than 4-potters -- because a twinscroll turbine is neither necessary nor beneficial in a 3-cylinder. In a 3-cylinder the back flow problem does not exist in the first place and the engine can use a cheaper and more efficient single scroll turbo!

Edited October 23, 2011 by dwightlooi

[dwightlooi]

Of course, all this 3 and 4 cyl forecasting will be for naught if someone figures out a way to make a synthetic, clean burning gasoline substitute from sunlight. ;-) (Assuming, of course, that Big Oil doesn't already have this secret and owns all the patents to it)

Dwight, I was curious if your expertise extends to the chemistry of gasoline. I know that gasoline is a mix of organic hydrocarbons and additives, but I've questioned for years is what compound actually makes gasoline into gasoline.

I know that Benzene, Toluene, Xylene can be removed from gas and its still gas. But what chemicals cannot be removed? Can your car run on only Butane? Or Isooctane?

Obviously, Ethanol is not quite a synthetic, gasoline substitute, as its water attracting properties and the idea we need like 10 more earths to grow all the raw materials to produce it.

Reason I bring this up is that I don't foresee a future where everyone's needs are met by 4 cyls... and I'd rather burn LPG, CNG or E85 than be straddled with a 4 cyl.

I am not a chemist. But in general, I don't think there is a particular molecular definition of gasoline. Gasoline is a blend of various long chain hydrocarbon liquids. Generally, a liquid that fits into volatility, combustibility, density and calorific value ranges compatible with contemporary spark ignition gasoline engines are called gasoline.

Gasoline is attractive because it is portable (liquid at winter through summer temperatures) and yield a high energy density. LPG and CNG have low energy densities per unit volume -- meaning that to go the same distance as a tank of gasoline that fits under the rear seats, you need to give up most of your trunk space for their tanks. Batteries are worse -- taking up just as much room if not more plus weighing a hell lot more and taking hours to recharge. Ethanol is only slightly worse, but is corrosive and its production in general are either negative yielding or barely breaking even (meaning more or just as much energy is put into its production as is yielded in ethnanol derived calories. Hydrogen is perhaps the worst -- the lowest density gas in the universe or a -423 deg liquid. Gasoline is attractive -- along with diesel -- because despite complaints about oil prices and despite the myth of global warming, they are the cheapest, most readily available and most portable choice for fueling passenger vehicles.

The time for alternative fuels will come in 50~100 years when the progressive depletion of easily accessible fossil fuel reserves drive prices to a level where alternatives become sufficiently attractive from a comparative standpoint. It will not come because misguided politicians drinking global warming coolaid wants to push "green" energy or because certain minorities within the public wants to feel good about themselves. Countries which chooses to shoot themselves by not using the most affordable and available fuel, but instead seek alternatives before their time or control non-issues like carbon emissions will be doing just that -- shooting themselves in the foot economically and nothing more. Because of the portability of combustible hydrocarbon liquids, the natural progression is for us to move from easily accessible oil fields to less accessible oil fields (deep water, mountainous regions, polar areas) to reforming tar sands and coal into liquid fuel, then finally making hydrocarbons from expansive cultivation of biomass. At some point, probably around when we get to reforming tar and coal, things like ethanol will start to make sense for air transportation and batteries will start to become an economical compromise for passenger cars. However, wind, solar, hydroelectric, geo-thermal and all the "feel good" stuff combined will not meet more than 15~20% of the energy needs of today's human civilization much less tomorrow's. Unless you are willing to bet on returning to medieval lifestyles and the age of sail, there is only one viable energy future... NUCLEAR POWER distributed via the electrical grid and carried around in batteries. That is except for air and space crafts which will probably continue to use synthetically made combustible hydrocarbons.

The world's energy resources focal point at that point will switch from the middle-east to Australia where most of the Uranium are... not entirely a bad thing IMHO.