MILITARY VEHICLE TANK ENGINES: A TECHNICAL, PERFORMANCE AND CAPABILITY COMPARISON OF DIESEL VS LAND GAS TURBINE

Introduction

Armored fighting vehicle (AFV) effectiveness and survivability are in large part dependent upon the selection of a tanks engine. Various operational and technical factors come into play, but one of the most consequential descisions relates to the type of military engine technology slected – diesel or land gas turbine.

Engine requirements of AFVs differ significantly from engine designs used in civil applications. These differing requirements complicate the design appreciation process and require the development of specialist engines specifically to meet the demands of diverse military operational environments. Since many modern AFVs can weigh over 65 tonnes, it is accepted that engines need to produce in excess of 1000 kW in order to provide adequate mobility. The rule of thumb used is that the engine to vehicle mass ratio should exceed 20 hp/ton to prove a satisfactory level of automotive performance.

This report provides an overview of spark ignition and diesel engine technology and describes why most military vehicle manufacturers have chosen to produce AFVs that use diesel engines. This report then analyses the advantages and disadvantages of the gas turbine engines with comparisons drawn between the M1A2 Abrams, Challenger 2, Leclerc and Leopard 2 main battle tanks.

Diesel Dominance of Post WW2 AFV Design

There are three properties of combustion fuels that determine their suitability for use in an AFV. These consist of specific energy capacity per unit of volume, flammability and the manner of energy release¹. The specific energy capacity by weight of a fuel is often used to justify an argument for weight efficiency in commercial environments. Conventional commercial land vehicles need to carry less fuel as they weigh significantly less than an AFV. It is therefore appropriate for conventional land vehicles to specify effectiveness of a fuel relative to its specific energy capacity by mass. This is the case since conventional vehicles are not armoured and thus not constrained by the requirement to minimise their volume under armour. Minizing volume under armor is a critical design consider for armored fighting vehicles and such design approaches are driven by a fundamental understanding of the Iron Triangle guiding AFV design.

In order to meet statisfactory operational range requirements, AFVs must carry significantly more fuel, moreover, this fuel must be protected under armour to mitigate fuel exposion treats posed by different types of kinetic ammunition types. Reducing fuel volume for the same energy capacity can reduce the required amount of armour, its weight and increase a vehicle’s operating range. The table below illustrates that whilst the specific energy of diesel and petrol are very similar; diesel is superior due to its higher energy density. Energy density, not specific energy, is the desired characteristic for AFV applications.

Table 1 – Fuel energy data from IOR Energy

AFVs are designed to defeat threats prosed from a variety of chemical and kinetic ammunition types, such a high explosive (HE), armor piercing fin stablisizing discarding sabot (APFSDS) and high explosive squash head (HESH) rounds. SInce World War II, there have been two main design weaknesses related to AFV design; specifically fuel and ammunition storage. Penetration of the AFV fuel tank by chemical and kinetic threat rounds can cause catastrophic fuel explosions. One particualy benefit assocaited woth with the use of diesel fuel is that it has a significantly lower flash point than petrol and does not readily form a vapour at most ambient temperatures. This desireable chemical property of diesel makes it more stable and safer than petrol; history has now demonstarted that it is considerably safer to store diesel in an AFV than petrol.

Table 2 – Stability components of diesel and petrol

Diesel engines operate using different fuel ingnition principles to petrol engines. Whilst petrol engines require spark ignition to create combustion; diesel engines ignite fuel under compression due to the lower autoignition temperature of diesel (see table 2). Petrol engines have relatively lower compression ratios than diesel engines which gives them a performance advantage per unit swept volume, however, diesel engines perform better under part load conditions.  This feature of diesel engine operation is extremely important in driving down the logistic burden associated with AFV fleets gross fuel usage and resupply operations. Diesel engines can sustain a high degree of forced induction whereas petrol engines suffer from efficiency loss at proportionally similar induction values⁴. Whilst not providing the same peak torque and power performance characteristics of petrol engines (on an equivalent swept volume basis), diesel engines have many desireable operational and technical qualities that enhance their appeal for use if AFV applications. The most important relate to reduced fuel flammability and lower specific fuel consumption.

Whilst petrol engines may give greater performance per unit of swept volume, diesel engine fuels have a higher energy density, are safer to store and perform better at part loads or under forced induction. Shortfalls in diesel engine torque and power performance and now easy overcome by the use of high-presssure turbo-chargers, albiet as the cost of increased engine complexity, cost and reduced reliability.  As such diesel engines have become the preferred military engine choice to petrol engines in AFV design. Notable exceptions to this are the U.S. Abrams and Russian T-80 series of tanks.

Issues and Features of Gas Turbine Engines Relative to Diesel Engines

The technology to integrate gas turbine engines into AFVs first appeared in production tanks with the advent of the Russian T-80 in 1978⁵. Although gas turbines are an effective competitor to reciprocating diesel engines, a number of technical considerations must be explored first to understand the relative advantages and disadvantages of the two technologies. The fuel consumption, endurance, reliability and maintainability must be weighed against the power to weight ratio and power density delivered by the gas turbine

Gas turbines were incorporated into early 3rd generation tanks such as the T-80 and M1A1. This is due to gas turbine engines being able to deliver significantly higher power to weight ratios and very desirable torque backup characteristics over the full engine operating speed range compared to that of diesel engines of similar outputs. Modern supercharging and tubocharging packaging solutions have bridged the gap in recent years such that diesel engined tanks now have similar power to weight ratio to that of ground turbines in tanks (see table 3 below)

Table 3 – Tank power to weight ratios and service entry dates⁶.

Power density is the engine power produced per unit volume of the engine. Gas turbines have higher power densities which reduces the volume under armour used by the engine. This is a desirable outcome as it increases the useable space under armour for other requirements such as ammunition storage. Gas turbines that incorporate a recuperator to reduce fuel consumption have a similar power density to diesel engines⁷. As such gas turbine engines retain a power density advantage over diesel but at the expense of considerably higher fuel consuption values.

Fuel consumption is proportional to the weight of the vehicle and overall driveline mechanical efficiency. When considering a like diesel and gas turbine vehicle, the gas turbine consumes significantly more fuel as it is most efficient when operating at high rpm (see figure 1 below), whereas the diesel is considerably more fuel efficient at part loads (such as idle). Part load engine operation is an unavoidable part of routine AFV military ground operations as they spend considerable time operating in a stationary mode whilst conducting surveillance, reconnaisance and other tactical tasks.

Figure 1 – Gas Turbine Specific fuel consumption vs                                                engine load⁸

This causes gas turbine AFVs to have a limited operational dwell time or requires an auxiliary power unit to run ancillary systems whilst the engine is switched off.⁹ Although requiring more fuel, gas turbines have better torque backup characteristic than diesel engines particularly at low engine speeds. Whilst diesel engines produce torque drops away at low rpm, gas turbine engines torque increases almost linearly at lower rpm ranges (see figure 2).

Figure 2 – Gas turbine torque curve¹⁰

Gas turbines and diesel engines are considered to be similar in reliability when subjected to manufacturer specified maintenance intervals and procedures. Whilst the service intervals for gas turbines and diesel engines are comparable in civilian roles; the gas turbine is more sensitive to operation in adverse environments (such as deserts) due to its tighter tolerances and higher air intake rate¹¹. The regular inspection requirements of turbine engines are significantly more involved and can only be conducted by specialist personnel and specialist facilities. Replacement parts for turbine engines are significantly more expensive due to the aerospace nature of the exotic materials used to sustain high operating temperatures within the engine.

Gas turbines generate significantly less noise than diesel engines¹². Whilst a positive attribute, it is noted that most AFV noise is generated through the tracks and the motion of the vehicle itself. Gas turbine engines produce significantly more and higher temperature exhaust gas. This makes gas turbine engine AFVs more susceptible to detection and targeting by technologies using infrared optics and targeting by technologies using infrared optics within the targeting system. Consequently, gas turbine engines have an overall detrimental effect on the AFV survivability.

Implications of Engine Choice on Operational Effectiveness

The future operational effectiveness of heavy AFVs will be affected by the choice of engine technology as each has different advantages and disadvantages. The key technical issues worthy of consideration include power to weight ratio, through life maintainability, thermal signature, fuel consumption and power production.

Power- to-weight ratio. It is clear that gas turbine engines have a higher power density than diesel engines. For a given output, a gas turbine engine weighs less and takes up less volume under armour than its diesel counterpart. However this fact does not take into account the ancillary systems used by the gas turbine. Gas turbines require larger air intake assemblies, heat exchangers and auxiliary power units to enable dwell time. Furthermore, gas turbine equipped AFVs must carry significantly more fuel than a diesel AFV for the same operating range. Whilst the exact breakdown of these components is beyond the scope of this whitepaper, one can observe that the French Leclerc AFV maintains a higher power-to-weight ratio than the M1 Abrams (as depicted in table 3) whilst differing in weight by only 3 tonnes. With arguably little discernable power-to-weight ratio difference between heavy AFVs using either a diesel or gas turbine, it appears difficult to justify the additional fuel burden on the logistics chain borne by gound gas turbine technology.

Through-life maintainability. As discussed above, gas turbines are a significant maintenance burden for any land force. Gas turbines require more frequent, more detailed, more specialised and more expensive maintenance than diesel engines. This can limit operational availability for three reasons: 1) AFVs must be accompanied by a significant logistics footprint wherever they go, reducing operational flexibility, 2) lower mean operating time between services reduces track miles that can be accrued thus reducing available training time, and 3) higher maintenance and fuel costs reduce operational activity due to significantly increased operating costs.

Thermal signature. Gas turbines produce a higher infrared signature as discussed earlier. Since anti-tank weapons pose a significant battlefield threat, AFV equipped with a gas turbine are likely to reduce the overall vehicle survivability when compared against diesel equipped AFV.

Fuel Efficiency. Gas turbine engines have significantly inferior specific fuel consumption rates at part load (see figure 1). It is most efficient to run the turbine at maximum rpm at all times. Even at peak fuel efficiency, gas turbines are significantly less fuel efficient (in the order of 10-20%) when compared to diesel engine of comparable power output. Poor fuel efficiency is the most significant and detrimental factor associated with gas turbine engines that do not operate continuously at 100%. In fact the gas-guzzling, gas turbines used in the US Abrams M1 tanks during Operation Desert Storm (1991) are attributed to significantly slowing the operational advance of allied forces during the short nine day campaign.

Engine Torque. Gas turbines have more desirable torque backup characteristic as discussed in paragraph 11. This is particular beneficial to mobility performance when AFV need to operate at low speeds in marginal terrain or in support of dismounted infantry. This high torque provides excellent low-speed, tractive effort performance and thus greatly enhances vehicle mobility.  However, most of this operational advantage is largely negated by the use of diesel engined AFVs using skid steering transmissions fitted with hydro-dynamic torque convertors. Modern AFV powerpacks using German MTU-Renk engine-transmission combinations offer torque backup characteristics that almost match those of gas turbine units.

Recent Developments – Abrams M1E3

Conclusion

Gas turbines held a power to weight ratio advantage over conventional diesel engines from the late 70s to early 90s that saw their integration into AFVs like the T-80, M1A1 and M1A2. Diesel engines are not be able to produce power densities greater than that of gas turbine itself, but modern military diesel engines can be integrated into an AFV utilising a whole of system approach that can equal or better a gas turbine. Gas turbines have favourable torque backup characteristics but this must be weighed against increased IR signature, difficulties in maintainability and poor fuel economy. Whilst gas turbines have the capacity to produce higher peak performance, the throughlife requirements of an AFV must be considered in parallel.

Diesels perform effectively across a broad range of mission profiles, cost less to run and maintain, and have a smaller logistics footprint. Diesel AFVs are easier and cheaper to purchase and disperse amongst fighting brigades as they do not require as much specialist knowledge or equipment. As such, until the main drawbacks associated with gas turbine engines are overcome, diesel engines will remain the backbone of AFV formations.

Written by: Mark Eggler and Jesse Spiller

References:

1. Kenneth Macksey, Tank warfare : a history of tanks in battle, London : Hart–David 1971

2. Glenn Elert, Ignition Temperature of Gasoline, The Physics Factbook, 16 Jun 13

3. United States Department of Labor, Occupational Health and Safety Chemical Sampling Information, 16 Jun 13

4. Klaus, Handbook of Diesel Engines, Mollenhauer, 2010

5. Wilson, T—80U Main Battle Tank, FAS Military Analysis Network

6. wikipedia.org

7. Walsh, Fletcher, Gas Turbine Performance, Wiley & Sons, April

8. Retrieved from Honeywell.com

9. Rajput, Power Systems Engineering, Laxmi, 2006

10. Retrieved from www.chrysler.com on 16 Jul 13

11. Boyce, Gas Turbine Engineering Handbook, Elsevier,

12. Soares, Gas Turbines, Air, Land and Sea applications, BH publishing, April 2011

About the Author

Daniel Eggler, BE (Hons), PhD. 

Dr Daniel Eggler has over 9 years’ experience delivering educational excellence in the field of mechanical engineering.