Why Are Standard Diesel Engines Limited in Capacity Compared to Petrol Engines?

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Discussion Overview

The discussion revolves around the limitations in engine capacities of standard diesel engines compared to petrol engines, focusing on the design, weight, and operational efficiency of diesel engines in passenger vehicles.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant notes that standard passenger car diesel engines typically have smaller displacements compared to petrol engines, questioning the reasons behind this limitation.
  • Another participant suggests that diesel engines are designed for economy, which favors smaller displacement due to the higher weight associated with diesel engine components.
  • A claim is made that diesel engines must operate at higher compression ratios, which contributes to their increased weight compared to gasoline engines.
  • Historical context is provided regarding General Motors' failed attempts to convert gasoline engines to diesel, which affected the reputation of the Oldsmobile brand.
  • Concerns are raised about the weight penalty of diesel engines in passenger cars, as they require stronger components to handle higher torque outputs.
  • One participant questions whether weight is the sole reason for the capacity limitations, suggesting that modern diesel engines are often made from aluminum, potentially reducing weight differences.
  • Another participant explains that existing engine designs, such as BMW's 3.0L diesel, influence the decision to not develop larger engines, as it is more cost-effective to build upon existing architectures.

Areas of Agreement / Disagreement

Participants express differing views on the primary reasons for the limitations in diesel engine capacities, with some attributing it mainly to weight considerations while others question the sufficiency of this explanation. The discussion remains unresolved regarding the exact factors influencing engine design choices.

Contextual Notes

Participants reference historical examples and technical specifications, but there are unresolved assumptions regarding the impact of engine weight and material choices on performance and design decisions.

jnnx
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this is thing I'm thinking about for few years.

why standard passenger car diesel engines are so limited in engine capacities range?

if we take petrol engines, there are anything from 1.0 liter to 3.0l Inline 4 cylinders, 2.0-4.0l 6 cylinders (or even bigger in USA), 4-7l V8 etc.
but take diesels. I4 are only 1.4l-2.5l, 6 cylinders? 2.7l-3.0 liter. V8 3.6-4.2 ?

most diesel engines have cylinders 0.5 liter big, or somewhere really close to that.

there are few bigger engines in offroaders (up to 3.2l 4cylinder for mitsubishi, and I think 4.2 6cylinder for old toyota off road) but that are truck engines

why is that? why is easier for BMW to have 2 or 3 turbochargers on 3.0 I6, than producing 3.5 or 4.0l I6 ?

only thing I can think off is bad NVH for bigger diesels. but can that be? big cylinders should have lesser thermal losses and better efficiency. and that is really important nowdays.

any ideas?
 
Engineering news on Phys.org
Most diesel cars have been designed for economy of operation, which is produced by smaller displacement engines. Because a diesel engine must be designed to operate at higher compression ratios than a gasoline engine, the diesel engine, for a given displacement, will be heavier than a corresponding gasoline engine, and extra weight is a big factor in making a car less economical to operate.

Many years ago, General Motors tried to convert gasoline engine designs to diesel, but the result was a fiasco, as the resulting engine, although beefed up in key components, was not a durable unit over the long term.

http://www.popularmechanics.com/car...otive-engineering-failures-oldsmobile-diesels

Although the engines were designed for Oldsmobile, they were eventually shared with other GM divisions and found their way into various models of car and truck. The problems experienced by these engines damaged the reputation of Oldsmobile in particular, and this played no small part in Olds once having the best selling car model in the US to being phased out by GM entirely in the span of about 20 years.

Dodge trucks have been offered with Cummins diesel engines for many years, starting with a 5.9 liter model inline-6 unit. This engine by itself weighs almost 1000 lbs, so its use in a passenger car like a large sedan, which would weigh approximately 4000 pounds with a gasoline engine, would impose a significant weight penalty, as other components in the car, like the transmission and suspension, would have to be stronger to carry the additional weight of a diesel engine and to handle the higher torque output.
 
Bigger usually entails heavier.

Just be glad its not a K19 Cummins XD
 
jnnx said:
why is that? why is easier for BMW to have 2 or 3 turbochargers on 3.0 I6, than producing 3.5 or 4.0l I6 ?

any ideas?

In this case it's because they already have a 3l diesel 6 pot. So why would they want to develop a completely new engine?

Its far cheaper and easier from a powertrain integration perspective to develop bolt on architecture than effectively a clean sheet design.

BMW have had a trend to commonise powertrain components as much as possible for a while now.
 
SteamKing: so weight alone is the only reason? I'm not sure about it. and cummis is probably not ideal comparison. these engines are cast iron, and modern diesel engines are usually made from aluminium (head and block).
so there probably wouldn't be that much difference between 3l and 3.5l V6. let say it is 40pounds. that is not an issue in 4000pound sedan or 5000pound SUV
 

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