A look at what engines are currently being offered by auto manufactures would seem to reveal that the once completely segregated designs of diesel and petrol engines are now headed towards pretty similar territory.
Starting with diesel engines, in the not too distant past – compression ratios were high (22+:1) in order to attain sufficient temperatures for compression ignition, this, along with the fact that low pressure injectors didn't do the best job of charge mixing consequently meant they were quite harsh, noisy, and overbuilt. With injector material advances, electronic control, and improved fuel qualities, common rail injection systems have improved in-cylinder charge mixing, resulting in lower compression (16-18:1), lower peak pressures, and structuraly lighter engines. Now, as it stands with 4th generation common rail, combustion is that smooth that engine blocks can even be made from cast aluminium, as more commonly done on gasoline blocks.
Diesel combustion and technology has progressed light years in the last decade with the introduction and advancement of common rail injection systems, and it would appear that this route of increasing injection pressures and adding further events per cycle is approaching the point of diminishing returns. What is interesting however, is that engine designs are heading further and further into gasoline territory with 2-stage turbocharger systems – mostly in the name of extending the engine torque curve, and improving bottom-end response. BMW has to be a field leader with this, first with their 2.0 twin turbo with a modulated 2-stage steup, and now the tri-turbo diesel.
Conversely, looking at emerging trends in gasoline engines, there seems to be a shift to downsized turbocharged direct injection engines, namely the fiat group twin-air, and the ford focus 1.0. From an efficiency and emissions standpoint, this idea makes sense - reduce and engines size to reduce friction, inertia and pumping losses, whilst also running the smaller engine at a more efficient running point more of the time. Once again it is a game chaining technology that has been the key enabler - gasoline direct injection seems to be on par with common rail developments in the diesel engine. Direct injection improves in-cyclinder charge mixing, enabling higher compression ratios and higher boost pressures. In it's second generation, GDI is even headed towards centralised injectors - similar to that in a diesel engine.
While downsized engines represent a practical concept in both engine and overall vehicle efficiency improvements, it is ironic to see that the torque curves from these engines is somewhat narrower (just like a diesel engine) when compared with a non-boosted variant. Looking at several 1.6L current production European engines from for and the PSA group, the trend is pretty clear. Unfortunatley for clarity, I'm not too bothered to go and graph them separately on the same scale, so the manufactures graphics will have to do.
Since downsizing essentially demands more load from a smaller engine, and a steeper torque curve, this necessitates higher mean pressures in cylinders, and hence higher bearing, piston, con-rod, etc mechanical loads. Interestingly lessons are being borrowed from diesel engines here in rod design for stiffness and block construction (the new 1.0 Ford ecoboost uses a cast iron block). Components also experience higher thermal loadings owing to these higher combustion pressures, meaning other diesel technologies such as piston ring oil cooling are coming into vogue.
While looking at other measures pursued by manufactures in looking for efficiency in newer gasoline engines, it is interesting to note the trend in shifting to longer bore/stroke ratios, and longer l/r ratio’s for con-rods – trends that are heading towards conventional engine design for diesel engines - essentially these parameters mean a longer stroke, and slower piston speeds. For comparisons sake, we can use the same engines (and the 1.0 ecoboost) as used for the torque curves to look at the bore / stroke ratio:
PSA DV6 1.6 diesel = 0.85
PSA TU5 1.6 gasoline = 0.95
Ford ecoboost 1.6 gasoline = 0.97
Ford ecoboost 1.0 gasoline = 0.88
From these figures, we can see that the latest 1.0 engine is heading pretty close to diesel territory.
In terms of compression ratio, It would appear that in general engines are able to utilise 10% more compression than in port injected engines. Interestingly, it appears that Mazda may be pushing this a notch further at 14:1 for their SKYDRIVE-G engine series. Interestingly, their diesel (SKYACTIVE-D) drops compression to 14:1 - in a strategy similar to the Toyota Prius's 1.8, with valve timing matched to give a higher expansion ration than compression ratio - thus giving a longer duration combustion.
An overall architecture similarity comparison gives the following now common features in engine architecture shared between gasoline and diesel engines:
· 4 valvers per cylinder
· Central direct injectors
· Turbocharged
· b/s l/r ratios approaching
· throttle-less
· compression ratio’s approaching
· piston ring cooling
· torque curve shape
· engine block construction
Interestingly, while we are seeing a convergence in design of both major IC engine architectures, it seems the perfect mix of the two, HCCI (homogeneous charge compression inanition), is some while away from being realised. In theory, HCCI engines make the best of both worlds, homogeneous in-cylinder charge, coupled with compression ignition ensures the smoothest combustion process possible, however in practice this would require a variable compression ratio to cover the speed and load range encountered in automotive powertrains. HCCI has been the subject of much research, and I'm yet to see anything convincing, so it's a small irony that gasoline and diesel engines are converging towards it in production variants.
Starting with diesel engines, in the not too distant past – compression ratios were high (22+:1) in order to attain sufficient temperatures for compression ignition, this, along with the fact that low pressure injectors didn't do the best job of charge mixing consequently meant they were quite harsh, noisy, and overbuilt. With injector material advances, electronic control, and improved fuel qualities, common rail injection systems have improved in-cylinder charge mixing, resulting in lower compression (16-18:1), lower peak pressures, and structuraly lighter engines. Now, as it stands with 4th generation common rail, combustion is that smooth that engine blocks can even be made from cast aluminium, as more commonly done on gasoline blocks.
Diesel combustion and technology has progressed light years in the last decade with the introduction and advancement of common rail injection systems, and it would appear that this route of increasing injection pressures and adding further events per cycle is approaching the point of diminishing returns. What is interesting however, is that engine designs are heading further and further into gasoline territory with 2-stage turbocharger systems – mostly in the name of extending the engine torque curve, and improving bottom-end response. BMW has to be a field leader with this, first with their 2.0 twin turbo with a modulated 2-stage steup, and now the tri-turbo diesel.
Conversely, looking at emerging trends in gasoline engines, there seems to be a shift to downsized turbocharged direct injection engines, namely the fiat group twin-air, and the ford focus 1.0. From an efficiency and emissions standpoint, this idea makes sense - reduce and engines size to reduce friction, inertia and pumping losses, whilst also running the smaller engine at a more efficient running point more of the time. Once again it is a game chaining technology that has been the key enabler - gasoline direct injection seems to be on par with common rail developments in the diesel engine. Direct injection improves in-cyclinder charge mixing, enabling higher compression ratios and higher boost pressures. In it's second generation, GDI is even headed towards centralised injectors - similar to that in a diesel engine.
While downsized engines represent a practical concept in both engine and overall vehicle efficiency improvements, it is ironic to see that the torque curves from these engines is somewhat narrower (just like a diesel engine) when compared with a non-boosted variant. Looking at several 1.6L current production European engines from for and the PSA group, the trend is pretty clear. Unfortunatley for clarity, I'm not too bothered to go and graph them separately on the same scale, so the manufactures graphics will have to do.
 |
| 1.6 Ford / PSA 110BHP turbo diesel (3rd generation common rail, aluminum block) |
 |
| 1.6 PSA 110BHP N/A gasoline |
 |
| 1.6 Ford 150 PS turbo down-sized turbo gasoline |
While looking at other measures pursued by manufactures in looking for efficiency in newer gasoline engines, it is interesting to note the trend in shifting to longer bore/stroke ratios, and longer l/r ratio’s for con-rods – trends that are heading towards conventional engine design for diesel engines - essentially these parameters mean a longer stroke, and slower piston speeds. For comparisons sake, we can use the same engines (and the 1.0 ecoboost) as used for the torque curves to look at the bore / stroke ratio:
PSA DV6 1.6 diesel = 0.85
PSA TU5 1.6 gasoline = 0.95
Ford ecoboost 1.6 gasoline = 0.97
Ford ecoboost 1.0 gasoline = 0.88
From these figures, we can see that the latest 1.0 engine is heading pretty close to diesel territory.
In terms of compression ratio, It would appear that in general engines are able to utilise 10% more compression than in port injected engines. Interestingly, it appears that Mazda may be pushing this a notch further at 14:1 for their SKYDRIVE-G engine series. Interestingly, their diesel (SKYACTIVE-D) drops compression to 14:1 - in a strategy similar to the Toyota Prius's 1.8, with valve timing matched to give a higher expansion ration than compression ratio - thus giving a longer duration combustion.
An overall architecture similarity comparison gives the following now common features in engine architecture shared between gasoline and diesel engines:
· 4 valvers per cylinder
· Central direct injectors
· Turbocharged
· b/s l/r ratios approaching
· throttle-less
· compression ratio’s approaching
· piston ring cooling
· torque curve shape
· engine block construction
Interestingly, while we are seeing a convergence in design of both major IC engine architectures, it seems the perfect mix of the two, HCCI (homogeneous charge compression inanition), is some while away from being realised. In theory, HCCI engines make the best of both worlds, homogeneous in-cylinder charge, coupled with compression ignition ensures the smoothest combustion process possible, however in practice this would require a variable compression ratio to cover the speed and load range encountered in automotive powertrains. HCCI has been the subject of much research, and I'm yet to see anything convincing, so it's a small irony that gasoline and diesel engines are converging towards it in production variants.
interesting. Actually the torque curve and narrow band would suite both a CVT and Fuel Electric designs (gosh I don't want to say Hybrid). I use a Yamaha with a CVT variator and its screamingly good. My mates DN-01 with an electronic varation on that variator works substantially better. So perhaps the only issue we have with this is the lack of good gearbox design and perhaps our love of 'stirring the stick' in driving.
ReplyDeleteGlad you mentioned the gearbox as being the weak link - I'm yet to hear anyone in the engine industry talk about it!
ReplyDeleteI do wonder if a CVT would be up to the task re transmission weight / torque ability for a car - I seem to recall that someone tried it a long while back - does warrant further investigation.
Aside from that, It's interesting how the French and Italians are leading the way in CO2 emissions, yet most of their cars still have a standard 5spd manual - while the Germans are a while off emissions, they are into 7-8 speed auto transmissions and twin clutch manuals - but still a while off CO2 emissions due to vehicle weight.
The weight and torque physics is a significant issue isn't it. But perhaps the solution is that we produce cars with less weight (and perhaps thus requiring) less torque? Do we need it?
ReplyDeleteSure people will want their 5L-V8 combos for showing off and other bits, but perhaps for practical transport we can go back to thinking economically rather than only extravagantly ...
nah ... never happen