Economy: Small-capacity turbo engine vs larger NA
Discussion
Can someone here please explain something to me?
We hear all the time that small(er)-capacity turbo engines are going to be the way forward, for normal cars up to even the fastest supercars and F1 in 2013. SWMBO's Alfa Mito has a 1.4 turbo that biffs out 155bhp, and very fun it is too. The stated intention is to reduce CO2 emissions, and whether or not you believe in anthropgenic global warming there's nothing wrong in increasing the efficiency of an engineering solution.
However, I don't quite understand how there is any reduction. Let's take two examples:
- Engine 1 is a 2-litre I4 with forced induction that produces 300bhp at 6000rpm.
- Engine 2 is a 4-litre normally aspirated V8 that produces 300bhp at 6000rpm.
Now, as the power produced by an engine is inherently linked to the chemical energy contained in the petrol being put into it (less friction and other efficiency losses), we can assume that at a given engine speed, the same amount of petrol is entering the cylinders, going foom and making nice noises. In order for combustion to happen properly, the same air:fuel ratio must enter the cylinders and to fit the same volume of air into the smaller cylinder volume, forced induction is required for the smaller engine.
So far so obvious. So... if the same amount of fuel is being burnt, surely the same amount of CO2 is produced at any given engine speed?
We hear all the time that small(er)-capacity turbo engines are going to be the way forward, for normal cars up to even the fastest supercars and F1 in 2013. SWMBO's Alfa Mito has a 1.4 turbo that biffs out 155bhp, and very fun it is too. The stated intention is to reduce CO2 emissions, and whether or not you believe in anthropgenic global warming there's nothing wrong in increasing the efficiency of an engineering solution.
However, I don't quite understand how there is any reduction. Let's take two examples:
- Engine 1 is a 2-litre I4 with forced induction that produces 300bhp at 6000rpm.
- Engine 2 is a 4-litre normally aspirated V8 that produces 300bhp at 6000rpm.
Now, as the power produced by an engine is inherently linked to the chemical energy contained in the petrol being put into it (less friction and other efficiency losses), we can assume that at a given engine speed, the same amount of petrol is entering the cylinders, going foom and making nice noises. In order for combustion to happen properly, the same air:fuel ratio must enter the cylinders and to fit the same volume of air into the smaller cylinder volume, forced induction is required for the smaller engine.
So far so obvious. So... if the same amount of fuel is being burnt, surely the same amount of CO2 is produced at any given engine speed?
Forced induction allows you to vary the amount of air in the cylinders to a greater degree than NA.
Less air in the cylinders means less fuel needs to be added to match, resulting in better economy and emissions.
Yes, they will both be using roughly the same amount of fuel at full throttle, but how often are you actually at full throttle on your typical journey?
Less air in the cylinders means less fuel needs to be added to match, resulting in better economy and emissions.
Yes, they will both be using roughly the same amount of fuel at full throttle, but how often are you actually at full throttle on your typical journey?
snotrag said:
Gizmo! said:
(less friction and other efficiency losses)
You can't just do that... especially in a discussion about why one engine is more efficient than another!By that logic big single-cylinders are the way forward?
the fundamental answer is that in the real world (and even more so over the EUDC test drivecycle) you will not be using anything like the peak output of the typical passenger car engine!
(typical mid sized family car requires about 15kW to do 50mph, but most engines now make well over 100kW peak)
So, for all the time you aren't using 100kW, the smaller, lower friction engine with a higher thermal efficency can make the 15kW required with less fuel.
(typical mid sized family car requires about 15kW to do 50mph, but most engines now make well over 100kW peak)
So, for all the time you aren't using 100kW, the smaller, lower friction engine with a higher thermal efficency can make the 15kW required with less fuel.
Currently small displacement turbo engines are very good in the fuel and emissions tests set by the EU and in part by some set by the EPA in America.
This isn't and likely won't always be the case. And car makers usually go to lengths to achieve better figures in these tests.
e.g. the C4 Corvette, it has a 4+3 speed manual gearbox originally. A 4 speed with an automatic OD on the top 3 gears. This allowed it to avoid the "gas guzzler" tax that most performance cars where subject too, yet still retain the performance.
I know engines it's a bit different, but it's a similar concept. Afraid I can't answer your question directly though.
This isn't and likely won't always be the case. And car makers usually go to lengths to achieve better figures in these tests.
e.g. the C4 Corvette, it has a 4+3 speed manual gearbox originally. A 4 speed with an automatic OD on the top 3 gears. This allowed it to avoid the "gas guzzler" tax that most performance cars where subject too, yet still retain the performance.
I know engines it's a bit different, but it's a similar concept. Afraid I can't answer your question directly though.
Should have added that a lower number of cylinder has less friction (piston ring sliding friction makes up ~75% of engine reciprocating friction!) and smaller combustion chamber that are filled with a higher density of charge (forced induction) have a more favorable volume to area ratio (so less heat is lost to the cooling system (heat not lost equals pressure in a sealed system which equals >force on piston and hence >torque)
However, ay high outputs, the large capacity NA engine, with a better spark efficiency and a higher thermal limit (low EBP = low EGT!) will probably be more efficient in terms of BSFC than the downsized engine (but we don't actually drive round at WOT peak power much in reality!)
However, ay high outputs, the large capacity NA engine, with a better spark efficiency and a higher thermal limit (low EBP = low EGT!) will probably be more efficient in terms of BSFC than the downsized engine (but we don't actually drive round at WOT peak power much in reality!)
Lower capacity generally means lighter, lighter engine can also mean lighter chassis. I suspect that the saving is here.
You're also comparing the maximum output of both engines, which will be at WOT. On the turbo, this means maximum boost. Off throttle, the turbo'd lump will be off boost, saving considerable fuel.
But you're right, put these two engines in the same car, drive them at the same speed and consumption should be very similar.
You're also comparing the maximum output of both engines, which will be at WOT. On the turbo, this means maximum boost. Off throttle, the turbo'd lump will be off boost, saving considerable fuel.
But you're right, put these two engines in the same car, drive them at the same speed and consumption should be very similar.
Max_Torque said:
Mikeveal said:
But you're right, put these two engines in the same car, drive them at the same speed and consumption should be very similar.
only if that same speed requires full engine output......Not to forget that most manufacturers' output, fuel consumption figures etc are obtained in clinical laboratory conditions NEVER achieved in the 'real' world.
Reality is a whole different kettle of ballgames. My two litre Turbos use less fuel than their normally aspirated identical models using the same engine for given useage. I did not believe this at first but, careful measurement confirmed this. It's marginal but, there all the same. Is it the cars or my driving that plays the larger part in these things. I expect its mainly the way the things are driven.
Reality is a whole different kettle of ballgames. My two litre Turbos use less fuel than their normally aspirated identical models using the same engine for given useage. I did not believe this at first but, careful measurement confirmed this. It's marginal but, there all the same. Is it the cars or my driving that plays the larger part in these things. I expect its mainly the way the things are driven.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff