Brand New Combustion System -- Future of the Pushrod V8

[dwightlooi]

GM has claimed that along with direct injection, the upcoming Gen V Small Block V8 will feature a (quote) "brand new combustion system". This has left many guessing at what that is supposed to imply. The common speculations are:-

• Cam-in-cam independent VVT
  
• Variable Valve Lift
  
• Advanced version of Cylinder Deactivation
  
• Some combination of the above
  

But, I think we may all have overlooked the obvious...

There has been much talk about the possibility -- some even say likelihood -- that the next generation V8s will see a downsizing of displacement rather than improvements in performance. However, reducing displacement is not nearly as effective as going to an Atkinson Cycle in decreasing the brake specific fuel consumption. Put simply, reducing the displacement of a 6.2 liter V8 by say 30% to 4.3 liters will yield less fuel economy benefits than keeping the static displacement at 6.2 liters and simply delaying the closure of the intake valves such that 30% of the compression stroke is negated. The effective displacement will still be 4.3 liters and the power output will be similar to a 4.3 liter engine, however fuel economy will be BETTER than a 4.3 liter engine. This is why the Prius uses 1.8 liter Atkinson Cycle engine rather than a "normal" (Otto Cycle) 1.3 liter engine.

This begs the question... perhaps, the Gen V Small block will be an Atkinson Cycle powerplant? Perhaps the mainstream displacemet will actually grow in displacement from 6.2 to 7.0 liters while it adopts the Atkinson Cycle? Such an engine will make approximately 375 hp / 350 lb-ft -- roughly equivalent to an Otto Cycle 5.0 liter. Not exactly spectacular, but it will be decent enough to provide performance parity with the C6 if they can shave 250 lbs off the car. The engine will exceed a bi-turbo V6 in terms of fuel economy while tipping the scales at a lower weight. The pushrod design's superior packaging density and lower weight actually compensates for the Atkinson engine's biggest flaw -- poor specific output and lousy power density (by size or weight). It'll still have the smoothness and burble of a crossplane V8. And, a car that makes 60 mph in 4.2 seconds while turning in around of 23/32 mpg all without the weight, cost and complexity of hybridization? It doesn't sound bad actually.

Can the Future of the V8 be an Atkinson one?

Edited March 26, 2012 by dwightlooi

[regfootball]

sounds good but all the auto rags would rip on it for low power / displacement. they would say you have 7 litres why not maximize the hp.

the flip argument is you could design an all new block around smaller displacement and borecenters etc and it would make the packaging even more efficient.

[CanadianBacon94]

maybe have the otto cycle 7.0 for the zo6 the next year

[G. David Felt]

I would think you could take this approach plus use other technologies to come up with a combined package of fuel efficiency plus power that would negate the bloody moron auto mag writers.

[Drew Dowdell]

With an effective VVT system, can't you get both Otto and Atkinson cycles out of the same engine? What about HCCI? We know it is getting direct injection which is a prerequisite for HCCI. I know there were some NHV issue in the 4-cylinder HCCI that GM was playing with, perhaps going from inline 4 to V8 could negate some of those...

[Camino LS6]

Interesting, but I can't say that I'm quite ready to sign on to the idea.

[dwightlooi]

With an effective VVT system, can't you get both Otto and Atkinson cycles out of the same engine? What about HCCI? We know it is getting direct injection which is a prerequisite for HCCI. I know there were some NHV issue in the 4-cylinder HCCI that GM was playing with, perhaps going from inline 4 to V8 could negate some of those...

A VVT system, no. If you want the ability to switch between Otto and Atkinson cycles, what you need is either a cam switching system (ala VTEC) or a continuously variable valve duration control system. Basically, you need the ability to change the duration of the intake valve opening period. Long duration = Atkinson, short duration reverts to Otto. You can do it in steps or you can do it continuously.

A by product of this however is that when you go from a 30% compression stroke negation to a regular Otto cycle, you also raise the compression ratio by roughly 43%. An engine with a 10:1 effective compression ratio in Atkinson mode will shoot to 14.3:1 when it goes to full Ooto cycle mode . You need to be able to deal with that. Alternatively, you can go light on the Atknson mode with say a 20% negation. This gives you a 12.5:1 compression in Otto mode on a 10:1 effective compression in Atkinson mode. Fuel economy wouldn't be quite as good, but it'll still be better than the current 6.2. On the otherhand, at full boil in Otto mode, this motor will make a whoping 530hp. Even in Atkinson mode its good for ~425 hp which is close to the current LS3. By the way, this has already been done. A 20% negation Atkinson at low loads and full Otto Cycle under full load is exactly what the current Honda Civic's D18A engine is doing with its iVTEC system. Part-time Atkinson operation along with a 2,700 lbs curb weight allow the Civic to clock in at 29/41 mpg using a 1.8 liter 140hp powerplant with a 5-speed auto and no hybridization -- beating the Cruze's Eco's 138hp 1.4 liter Turbo with a 6-speed auto at 26/37 mpg.

[Robert Hall]

Not interested in speculating, would like to see some concrete facts about the Gen V engine...I assume it will debut with the C7 Corvette next year?

[dwightlooi]

Not interested in speculating, would like to see some concrete facts about the Gen V engine...I assume it will debut with the C7 Corvette next year?

The Gen V Small Block facts we know thus far are:-

• It will be a 90 deg Vee engine with pushrods and 2 valves per cylinder
  
• It will use the traditional 4.4" small block bore spacing
  
• It will be aluminum block and heads
  
• It will have Direct Gasoline Injection
  
• It will have Variable Timing (Sychronous or Independent is unknown at this point)
  
• It will have Cylinder Deactivation (at least on some versions of the engine)
  
• It will be assembled in Towanada, NY and St Catherines Canada
  
• It will get its engine block castings from Defiance, OH and Bedford, IN
  
• It will be introduced next year in the 2014 model Corvette
  

[G. David Felt]

EXCITEMENT!!! I am very excited by what I see as a push to finally revolutionize the engines.

[loki]

do we know when "this" engine will be intro'd? LA late this year or Detroit next?

[dwightlooi]

do we know when "this" engine will be intro'd? LA late this year or Detroit next?

In time for the C7 Vette for sure. Then perhaps the ATS-V and/or the SS. So, I'll say 2013.

[dwightlooi]

Let me refine this concept further...

6.8 liters, 439 hp, 20 / 32 mpg

In contrary to popular rumors that the Gen V Small Block V8 will be downsized in displacement, I believe that there is a possibility that it will actually be upsized. I am not saying this because I believe that GM is flagrantly going to disregard any fuel economy concerns or because the designed power output for the next corvette engine need to exceed 500 hp (naturally aspirated). I am saying this because the most economical 450 hp may not be a 5.5 liter V8 but a 6.8 liter V8. Sounds ridiculous? Well, read on and you will see that it actually isn’t.

There are two concepts I want to discuss here. The first is Mass Efficiency.

You’ll notice that a 4.8 liter Small Block V8 is not significantly (if any all) lighter than a 6.2 liter. This is because when you change the bore diameter and stroke length without altering the bore spacing the block and heads are roughly the same size and the cylinder walls are actually thicker. This applies to all engine families, in all configurations, from all makes, not just to Smallblock V8s. Hence, the most mass efficient engine tends to be the one that stretches displacement as much as possible given bore spacing and external dimensions. In short you want the engine to be as big on the inside as possible for a given exterior size. This leads to the best power to weight ratios and power to size ratios. Traditionally this has been a strong suit of pushrod designs and a weak point of DOHC designs.

Why 6.8 liters you may ask? Because this is the displacement you arrive at with the LS3’s bore dimensions which is thick enough to accommodate supercharged applications and the LS7’s stroke which is the longest GM could squeeze into an LS block. 103.25 x 101.6 gets you 6.8 liters out of eight pots.

The second concept here being that adopting "Atkinson Cycle" is more effective at reducing brake specific fuel consumption than reducing engine displacement.

Some of you may be familiar with the term “Atkinson Cycle”. The Prius, Fusion Hybrid, Volt and many other efficiency focused cars use “Atkinson Cycle” engines. To put it simply, an “Atkinson Cycle” engine is simply one which closes its intake valves very late causing part of the intake charge to be pushed back out the cylinders during the piston’s upward stroke. “Atkinson Cycle” engines achieve two things by using this mode of operation. Firstly, they reduce the effective displacement. If you kick 30% of the intake charge out you are effectively aspirating and burning fuel like an engine with 70% the displacement – you make less power and burn less fuel. Secondly, and perhaps more importantly, it makes the compression stroke 30% shorter than the power stroke; or the power stroke 43% longer than the compression stroke depending on how you look at it. This allows a greater expansion ratio and hence better energy extraction from each drop of fuel burned. Its like having a longer barrel in a rifle, the same power charge and the same bullet gets to a higher velocity and higher kinetic energy because there is more bore room and time for the expanding gases to accelerate the projectile.

The Prius for instance uses a 1.8 liter Atkinson Cycle Engine making a paltry 98 hp. That same 1.8 liter engine in regular Otto Cycle trim makes 140hp and Totota’s 1.3 liter engine makes 99hp. Why will the Prius – a contender for the fuel economy crown – use a 1.8 liter Atkinson instead of a 1.3 liter regular engine? Because a 1.8 Atkinson is more economical than the 1.3. It’s that simple!

A 6.8 liter Gen V V8 would normally make about 517hp (assuming a modest 5hp/liter specific output improvement over the LS engines due to Direct Injection). If we run an Atkinson Cycle cam on it that keeps the intake valves open ~30% into the compression stroke we end up with a 362 hp V8. But this 362hp 6.8L V8 will be more economical on fuel than a 4.8 liter V8 with the same output in the same manner that the 1.8 Toyota 2ZR-FXE (Atkinson) is more economical than the 1.3 Toyota 1NR-FE engine of the same approximate output. That’s not the end of the story, because with Cylinder Deactivation things get even more interesting… you see the Gen V engine will only need to strive for maximum efficiency during low load and cruise operations. That’s how you improve EPA MPG stats and turn in real world economy when people don’t drive with a lead foot. When somebody is burning tires, trying for a 1/4 mile record or simply trying to pass a semi on a 2-lane highway, trust me, they are not interested in fuel economy during that burst of acceleration. Hence, only the 4 cylinders that does not get deactivated by AFM needs an Atkinson intake cam lobe, the other four can have a full performance Otto Cycle cam. The resulting engine designed with 4 Aktinson cylinders and 4 Otto Cylinders will have about 439hp and all the fuel economy benefits of the 362 hp full Atkinson engine featuring cylinder deactivation – at least will when accelerating judiciously and while cruising.

How does a 439 hp 6.8 liter Direct Injected Gen V V8 with 20 / 32 mpg sound? Pretty Interesting to say the leastand roughly where the C7 Corvette needs it to be. All of the above is also consistent with the official GM claim that the new Gen V V8 will feature a “Brand new combustion system” – this is brand new indeed.

Edited August 3, 2012 by dwightlooi

[SAmadei]

Oh, goody, we're finally getting cold fusion!

[dwightlooi]

Oh, goody, we're finally getting cold fusion!

Not cold fusion, just a 130 year old engine operating cycle.

If you are wondering where the 20/32 mpg comes from, well it's an educated guess based on the following:-

• The Corvette (3250 lbs) already gets 16/26 mpg today with the 6.2 liter port injected LS3 engine.
  
• Direct Injection, Higher Compression and other improvements on the Gen V V8 should produce a 1 mpg improvement
  
• An expected 200 lbs reduction in vehicle weight, better aerodynamics and taller gearing is good for another 1 mpg increase
  
• We are already at a decent 18 / 28 mpg for a 6.2 liter Gen V V8 with the above changes
  
• Now, cylinder deactivation cuts the operating displacement in half, Atkinson cycle further reduces that by 30% and improve energy recovery from each fuel-air charge.
  
• The net result should be similar to using a (6.8 / 2 * 0.7) = 2.4 liter 4 cylinder engine on a Corvette, for which a 10~15% reduction in fuel consumption is a reasonable guess. 20/32 mpg is 11% and 14% better than 18/28 MPG respectively.
  

No Cold Fusion required.

[G. David Felt]

Oh, goody, we're finally getting cold fusion!

Not cold fusion, just a 130 year old engine operating cycle.

If you are wondering where the 20/32 mpg comes from, well it's an educated guess based on the following:-

• The Corvette (3250 lbs) already gets 16/26 mpg today with the 6.2 liter port injected LS3 engine.
  
• Direct Injection, Higher Compression and other improvements on the Gen V V8 should produce a 1 mpg improvement
  
• An expected 200 lbs reduction in vehicle weight, better aerodynamics and taller gearing is good for another 1 mpg increase
  
• We are already at a decent 18 / 28 mpg for a 6.2 liter Gen V V8 with the above changes
  
• Now, cylinder deactivation cuts the operating displacement in half, Atkinson cycle further reduces that by 30% and improve energy recovery from each fuel-air charge.
  
• The net result should be similar to using a (6.8 / 2 * 0.7) = 2.4 liter 4 cylinder engine on a Corvette, for which a 10~15% reduction in fuel consumption is a reasonable guess. 20/32 mpg is 11% and 14% better than 18/28 MPG respectively.
  

No Cold Fusion required.

If your right, you either have an inside line to info or the best damn crystal ball!

Time to pull up the chair and wait and see.

[Drew Dowdell]

Twin cam-in-block?

[dwightlooi]

Twin cam-in-block?

This is tricky for two reasons. Two cams (presumably one on top of the other) means pushrods at two distinct angles which complicates the rocker and channel placements. It also means additional friction for the additional cam and sprocket.

More importantly, it is completely unnecessary. Having two cams only has one tangible advantage -- being able to alter intake and exhaust timing independently. There is no other benefit for all the added hassles and packaging difficulties. If that is the goal, a cam-in-cam design (ala the Viper's 8.4 V10) meets that objective without incuring the penalties. The Viper engine is rated at 640 hp / 600 lb-ft from 8.4 liters. At the same specific output, a 6.2 liter Small block will deliver 472 hp / 443 lb-ft. And, that is without Direct Injection which is 100% confirmed on the Gen V Small block so it has the potential to do better.

Edited August 20, 2012 by dwightlooi

[Drew Dowdell]

Oh... no. That was me misunderstanding your earlier explanation. It sounded like you were proposing an atkinson cam and an otto cam.

[dwightlooi]

Oh... no. That was me misunderstanding your earlier explanation. It sounded like you were proposing an atkinson cam and an otto cam.

No, no. It's just one camshaft. 4-cylinders get the Otto intake lobes, 4 cylinders get Atkinson intake lobes.

The Otto cylinders get disabled when AFM kicks in, meaning that in its fuel saving mode the 6.8 V8 is running on four Atkinson cylinders and operate with an effective displacement of 2.38 liters -- because half the cylinders get all their valves shut off and the remaining is regurgitating 30% of the intake charge.

In full song, the engine is still running 4 cylinders in Atkinson mode and four in Otto mode. The Atkinon cylinders contribute about 181 hp, while the Otto Cylinders pitch in with 258 hp for a total of 439 hp. Essentially the same as the LS3.

A potential "performance first" version of the engine for the Z06 can then run with all eight with Otto lobes making 516 hp out of 6.8 liters. It'll still benefit from Cylinder deactivation, but the savings won't be as pronounced because the remaining cylinders are not operating in the efficient Atkinson cycle. Unlike the LS7 it won't rev to 7000 rpm and it won't use Titanium valves and other exotic materials (the AFM lifters can't go that high anyway). But it still makes 11 more horsepower from 0.2 liters less displacement from the increased compression and direct injection. Again, I try to err on the conservative side with estimates.

Edited August 21, 2012 by dwightlooi

[Drew Dowdell]

how about engine smoothness in Fuel Economy mode?

[dwightlooi]

how about engine smoothness in Fuel Economy mode?

I'll be like the current L99 V8 running on 4-cylinders in AFM mode just less powerful in that mode because of the Atkinson cycle operation.

[NeonLX]

I shoulda read this thread before making my dumb@ss comment in the Cruze Eco thread...

[G. David Felt]

I shoulda read this thread before making my dumb@ss comment in the Cruze Eco thread...

Thats OK, we all have one of those Blonde Moments every so often in life.

[NeonLX]

Yeah, but I'm a redhead...at least I used to be, back when I had hair on the ol' dome...

[dwightlooi]

You know what will be really interesting?

Type: 2.5L Atkinson Cycle Inline-3 w/ single balancer shaft

Construction: Aluminum Block and Heads

Bore x Stroke: 103.25 x 101 mm

Displacement: 2537 cc

Compression: 15.5:1

Valve Train: SOHC 2-valves/cyl (6-valves) with cam-in-cam dual VVT and Cylinder Deactivation

Fuel Injection: Direct Gasoline Injection

Power: 135hp @ 5800 rpm

Torque: 126 lb-ft @ 4800 rpm

Max Engine Speed: 6000 rpm

Fuel Requirement: 87 Octane unleaded

It'll be a very nice engine for compact cars like the Cruze, the Sonic or as the generator drive for the Volt or ELR.

Edited October 3, 2012 by dwightlooi

[dwightlooi]

Pair it transmissions like these and it gets even more interesting...

GM Electramatic 9E30 -- Hybrid Drive

Type: Transverse, FWD, Torque converter automatic transmission

Input Torque rating: 130 lb-ft

Maximum Input speed: 6000 rpm

Speeds: 9-speed

Ratio Spread: 8.0:1

Flywheel Integrated Motor-Generator-Starter (FMG): 27 hp @ 3000 rpm, 94 lb-ft @ 0 rpm (DC Motor), 6000 rpm Maximum Motor Speed

Electric Storage: 0.8 kWh -- 115.2V Lithium-Iron-Phosphate Battery

GM Hydramatic 9T30 -- Conventional Drive

Type: Transverse, FWD, Torque converter automatic transmission

Input Torque rating: 130 lb-ft

Maximum Input speed: 7000 rpm

Speeds: 9-speed

Ratio Spread: 8.0:1

Flywheel Integrated Alternator Starter (FAS): 0.9 kWe Alternator-Starter

Electric Storage: 0.08 kWh -- 13.2V Lithium-Iron-Phosphate Battery

[Guest Larry]

Questions.

On an 8 cylinder engine we run 1-3-5-7 all the time. 2-4-6-8 can be shut down. We have 2 options. We shutdown the fuel injector then open the valves or close them.

I don't know of any type of mechanism other then a cam lobe which could keep the valves open so I would assume we are just closing them.

If we remove oil from the lifer and the valves stay closed, we now have a pneumatic spring with no combustion or exhaust cycle.

1) What is the compression loss of this arrangement? Since we compress creating heat then decompress creating cooling is the loss low? Is the loss nothing much more then the friction of the system? If we were to allow every lifer to float and were to spin the motor by hand with a wrench on the crank it would still take a lot of effort. So this type of setup must have a reasonable amount of loss.

2) Are the mechanisms which controlled the pressure to the lifter durable. Will they last or will repairs eat up any fuel savings?

3) If the injector is turned off we have not lubrication to the upper rings since we have a zero lean situation. Is the oil spray from the crank enough in this case?

larryinutah

[dwightlooi]

Questions.

On an 8 cylinder engine we run 1-3-5-7 all the time. 2-4-6-8 can be shut down. We have 2 options. We shutdown the fuel injector then open the valves or close them.

I don't know of any type of mechanism other then a cam lobe which could keep the valves open so I would assume we are just closing them.

If we remove oil from the lifer and the valves stay closed, we now have a pneumatic spring with no combustion or exhaust cycle.

1) What is the compression loss of this arrangement? Since we compress creating heat then decompress creating cooling is the loss low? Is the loss nothing much more then the friction of the system? If we were to allow every lifer to float and were to spin the motor by hand with a wrench on the crank it would still take a lot of effort. So this type of setup must have a reasonable amount of loss.

2) Are the mechanisms which controlled the pressure to the lifter durable. Will they last or will repairs eat up any fuel savings?

3) If the injector is turned off we have not lubrication to the upper rings since we have a zero lean situation. Is the oil spray from the crank enough in this case?

larryinutah

(1) The current AFM system collapses the lifters on all the cylinders being deactivated. Compression is as before the difference being that there is no fuel charge being ignited. If there is only ONE cylinder then the crank will settle at Bottom-Dead-Center (BDC) and the effort to spin it will be considerable. However, if two or four cylinders are being deactivated and the effort is about the same as the friction of the engine plus the effort to compress those cylinders which are NOT deactivated.

(2) GM's 1st generation of AFM lifters do not have a stellar reliability record. They get stuck in the collapsed position on a some trucks and some owners got them swapped out for non-collapsible lifters. They did improve them and the current production examples seems much better.

(3) Lubrication has never been an issue. It is no different from a normally running 4-cycle piston engine. The upper piston ring is NEVER lubricated by the fuel spray; fuel spray does not contain significant lubricants except in 2-stroke engines burning a pre-mix of oil and fuel, or if you have a dedicated oil jet. The upper piston ring comes into contact with the oil film on the cylinder wall as it travels downwards. The oil ring (lower most ring) never completely remove the film from the wall ahead of the upper two rings -- a sponge and some solvent would, an oil ring will never do that. This is enough to lubricate both the walls and transfer enough lubricants to the upper rings to provide some protection during the top 5~10% of travel where the wall is always "dry" because it's above the oil ring at Top-Dead-Center. Basically, you don't have a lubrication problem with AFM equipped engines, period. You have a lubrication problem only when the rings or seals are perpetually in contact with dry walls and never, ever, contact lubricate wall surfaces -- such as is the case with the apex seals of a KKM Rotary (Wankel) engine. In such cases you either burn a pre-mix of oil + gas or you have an oil injector like that which Mazda's 13B and 20B engines use.